Scenario A - 1- info not available to ALL sailors (those w/o VHF)
Scenario B - 1-
Scenario C - 3, RRS do not apply before warning
Scenario D - 1- info not available to ALL sailors (those w/o VHF)
Scenario E - 1- info not available to ALL sailors (those w/o VHF)
Scenario F - 1
Scenario G - 1 /3,  had the boat cleared the finish line and was therefore not "racing" - italiics"
Scenario H 1

John, 

Do your SIs stipulate that competitors shall monitor a competitor VHF channel?  
It seems implied that they would by your reference to that channel.

A+B: 3.  Particularly helpful for large fleets or large boats, or during a pandemic if RC needs to wear masks. But maybe establish at the competitors meeting.
C: 3. Rules of part 4 only apply while racing. And I doubt this would be considered bad practice at a youth event.
D+E: 2. While exempt under RRS 41c, I would advise against. It is generally unnecessary (signals alone should be sufficient), and If something changes you could wind up in redress. 
F: 2. I believe this would technically be exempt under RRS 41d, but RRS 61.1b requires informing to take place AFTER the boat finishes. So there is no need or benefit to doing this.
G: 2/3. While required byRRS 61.1b, there might be better ways to go about this. I would say this is valid as a backup option.
H: 1/2. Might be exempt under RRS 41d, but dont be the one to test this. It's unfair to the competitors who finish properly. The exception to this would be if you did something that made your own finish line bad, in which case I would suggest requesting redress (against yourself) for effected boats.

For those claiming that 41c does not apply, consider the following:
1. Look at WS Case 120. It is legal to use a phone to look up weather information on the internet; the information itself must be free, but equipment used to access the information does not need to be free. Shouldn't the same logic apply to VHF radios, especially since a boat is more likely to have that on board than a cellphone?
2. Is RC a disinterested source? Since these scenarios are unsolicited, wouldn't that exempt the boat under 41d?

Scenario A - RC hails boats that were OCS at the start via competitor VHF
Can you please give more detail about this?

The vast majority of these fall under the "disinterested source" exemption of RRS 41(d).  By the wording of RRS 41, the RC cannot break RSS 41 (A boat shall not . . . )

Does it make any sense at all that you could be accused of breaking a rule because you heard something without soliciting it?

I will address the "best practices" question, though.
A) Calling OCS boats on the VHF - personally don't like it because it adds another layer of responsibility at the worst possible time on the signal boat - and it has a tendency to make a fleet more aggressive.  However, if that's what the fleet wants, that's what they get.  Not subject to redress has to be in the SIs.
B) UFD / BFD Boats on the course - again, if the fleet wants it, they get it.  But it needs to be in the SIs, since the rules place no obligation on boats to withdraw if they are OCS.
C) This is absolutely OK and should not be an issue as long as the competitor is not racing.
D) Absolutely good practice - as long as the appropriate (and correct) visual / sound signals are made.
E) Not sure I would do this - I try not to use Sierra at all.  Frequently, it's a race against time to get the mark boat anchored properly and set up to take finishes.  I've had to abandon two races after the fact when a shorten course was botched.
F) This is bad practice - the VHF radio is for public announcements, not individual.  Additionally, the RC needs to be very sure the boat hit the mark.  Often the bow wave will push it out of the way and the boat doesn't touch it.
G) Again, bad practice, unless there is no posting post race (for example, on a weeknight series where boats disperse to different clubs / marinas after finishing).  I would attempt to contact them another way (phone / text / email) that's more private.
H) Bad practice - and I'll have a chat with the person so it doesn't happen again.

In general, announcements to the fleet as a whole are good practice; announcements to individuals (on a public medium such as the VHF) are bad practice.

I) While racing, competitor asks on VHF, "RC, can you tell me if Mark C is to be left to port or starboard?" No answer.  A minute later Race Committee, hails on VHF. "All boats, Mark C is to be left to starboard!"  (VHF is required by SI).

I must admit, I've always been uncomfortable with the greyness of the use of VHF by race committees w.r.t. RRS41.

Assuming that the SI's state that all boats must carry a working VHF and maintain a listening watch on the appropriate channel.
(Disclaimer - Radio announcements do not replace the use of visual and audio signals required under the RRS. They should only be used as an aid where appropriate.)

1. Breaks RRS 41
2. Does not break RRS 41, but is bad practice
3. Nothing wrong with this

Scenario A - 3
Scenario B - 3
Scenario C - 1 or 3. You have stated "well before the scheduled START", but not how well before. If it was before the Preparatory Signal, then this is okay. If it is at or after the Preparatory Signal, then boats are racing and it would be breaking RRS 41. It would have to be unsolicited information by the mark boat at this point and not asked for by the competitor.
Scenario D - 3
Scenario E - 3
Scenario F - 2 - Sailing is a self policing sport and left to the competitors to protest. If, however, there are no competitors (or judges) in the vicinity to have witnessed the incident, then the RC should protest after the boat finishes.
Scenario G - Most will say 2, but I have had a RC protest dismissed by an International Jury for not informing the boat on the water via VHF when they had finished but instead waited until I (RC) was ashore to inform them.
Scenario H - 1 - It's unsolicited, yes, but is the RC at the Finish a disinterested source? For me, no, they are interested in the boat finishing. So unsolicited or not, the RC at the Finish is not a disinterested source.

If all boats are required to carry a working VHF and maintain a listening watch on a specified channel, as long as the announcements are kept to that channel, the key thing to think about for me, is a boat going to gain a significant advantage by the RC announcement?

Sorry I was away for a bit. I should have put in the assumptions that there is no prohibition on carrying a VHF in any rule. Also, the RC published the competitor VHF. I believe Case 120 is very clear that with those provisions in place, any announcement is freely available to all competitors. 

So, knowing that, I'm thinking:

A - Nothing Wrong
B - Nothing Wrong, but maybe bad practice unless the fleet has asked for it. Ideally you would communicate this intention in the SIs.
C - Nothing wrong (well before sequence)
D - Nothing Wrong
E - Questionable practice, but may be helpful in explaining something "odd" if it is unusual to use this practice. For instance, "Attention all competitors, Attention all competitors, we are seeing the same dark clouds you see. To ensure you all make it home safely, we will be displaying flag Sierra at the leeward gate. This means that the finish will be between the gate marks and you should proceed to shore quickly after finishing."
F - Obviously, this would be something egregious the RC saw, likely not in the presence of other boats. For instance, the leader who wasn't near anyone plowed over the mark. I think that by the time you would be inclined to hail, the time has naturally "expired" to take a penalty as quickly as possible. At that point, why would you hail, they can't reasonably take a penalty. 
G - Possibly hail after boat has finished, but certainly find a mechanism to ensure the notification is made. 
H - Bad practice, but if not interested, did not break a rule. I think this fundamentally "messes with the game" and isn't an RC role. 

Basically I agree with Matt's answers but have some comments on Scenario C and E. 

I have had to shorten course a lot over the years, and boats have been given redress twice against me because I did not inform them early enough. In each case the PC was convinced by the boat's case that it would have sailed the last leg differently if it had known the race was being shortened before it got to the mark zone for the previous mark. So I now inform them as soon as I can, preferably before they get to the mark zone for the previous mark, however even if I can't inform them in good time I still shorten if I need to to save the race and live with the redress hearings. 

I personally agree with Matt's answer for Scenario C, but believe that it infringes rule 41 as it is not unsolicited and the boat is in the racing area. However I would be interested in whether the boat would be given redress if the information the mark boat gave was wrong and it affected the boat's position........

I think Matt's answers are spot on and I like Roger's observations on E .. though C is OK as long as it occurs before their prep as the preamble to Part 4 clearly states that Part 4 rules only apply to boats racing, and racing is italicized thus refers to def:Racing

Rob .. re:"Scenario C - 3, RRS do not apply before warning" ..

I'd disagree with that statement as Part 2's preamble, which details when its rules apply, isn't contingent upon a time relative to the "warning" .. rather other measures which do not include that term. Therefore parts the RRS can apply before the warning.  That said, Part 4 is specifically limited to boats Racing, so the time-boundary is a boat's prep-signal.

I  think Rob meant "RRS 41" does not apply before the warning signal

Likely, but thought since it stood uncorrected that it was important to make it clear.

Also, though “"RRS 41 does not apply before the warning signal” is also a true statement (because the warning signal comes before the Prep), it could also be misleading to those that are following along to learn as I know many forum members do. 

Just wanted to make it clear that the time-event that Part 4 keys on is the prep signal.