Well I’ll have a go Jon. It depends on the evidence presented in a hearing, for I assume the scenario occurred during fleet racing.

In your description you say:
“After rounding the bottom mark, Y was slow to trim her main and come up to close hauled.” and,
“B came up quickly and Y's boom came inside B's backstay.”

If Y was slow, it would seem that Y was not keeping clear, taking too much room. Going solely on the diagram, I would say that at position 3, Y was sailing in the room to which she was entitled. After 3 she could and should have sailed higher, so was not sailing in the room to which she was entitled between 3 & 4. (See rule 21)

In consequence, when Blue luffed between 3&4 Yellow was vulnerable and broke rule 11 without the protection of exoneration under rule 21.

Did Blue break rule 16 by not giving Yellow room to keep clear? The hulls did not touch and Yellow had been slow to trim the main and the boom protruded over Blue. 

Mmmmm, I’m inclined to think the Yellow broke 11&14 and is DSQ. Blue broke 14, but there being no damage Blue is exonerated under 14(b).

What do you think Jon?

Phil.

Jon, World Sailing Case 25 is a situation very similar to the one in your question and supports Phil's answer. 

Mark and Phil, I am not so sure. First of all we need more information about the size and the speed of the boats, as well as about wind and waves.
Secondly in Case 25 the windward boat was one length behind the mark and still not clearly changing course to sail to the next mark. In the scenario we have here Yellow, on length behind the mark is already clause hauled. If you look at positions 2 and 3 you can see that the starbord side of Blue´s hull is nearly (position 2) and exactly (position 3) vertical below the end of Yellow´s boom. I have strong doubts ( again, depending on the not yet really known circumstances) that Blue gave mark room to Yellow, as Blue´s postioning so so close to Yellow is barely seamanlike. And do not forget that there are different heeling angles between the boat to windward, which heels stronget than the blanket leeward boat - and this is part of giving room or mark-room which was Blue´s obligation. To make a long story short: more facts!
Willii

Building on Willi's point:

Yellow was on a close-hauled course within just over one boatlength of rounding the mark. That seems seamanlike, given that Blue didn't give room for a wide-then-tight rounding (since Blue's not obligated to do so under 18.2.a).

A turning radius depends on the boat type and wind strength. Are there guidelines for this, at least for popular clases of boats?

At position 3, Yellow might have been able to turn quicker by heeling her rig to leeward to facilitate a turn and/or by steering harder. But that could have lead to rigs making contact and/or sterns touching.

Blue seems to want it both ways, 2 times:

1) Give minimal room so that Yellow rounds tight-then-wide, but expect Yellow to exit the rounding as if Yellow did a wide-then-tight rounding.

2) Expect Yellow to have a very tight turning radius, but don't her room to execute such a tight turn.

If Yellow took two or more boatlengths, then Blue would have a case. But turning up within a boatlength is very standard.

Further, at position 2, Blue's shrouds are within inches of Yellow's shrouds, and Yellow passed only a couple of feet from the mark. Blue was not leaving any buffer to avoid a collision with Yellow. And it was "reasonably possible" (Rule 14's words) for Blue to create a buffer simply by bearing away just before position 2, and keeping more of a gap between herself and Yellow.

For all these reasons, my recommendation would be to DSQ Blue under Rule 14.

More facts coming! Lots of good food for thought here - in order to answer some of your questions I've gone back to what video footage I have of this incident.  Blue had a gopro on the backstay - so, this new diagram is based on that as best I can make out...  these are Stewart 34's, fleet racing.  The racing is generally tight - both in the competitive sense and in the room given for manoeuvres.  On this day the water was flat and wind was probably 8-10kts.

A couple things to note about the new diagram:
This is much more representative of the relative room positions throughout. 
P3.  Y had trouble dropping her spinnaker and so was being pushed down.
P4.  B was dialed up and her skipper had a heavy helm.  Y had main luffing and spinnaker hourglassed and backwinded.
P5.  Contact, as described above, Y's boom inside B's backstay.  Hulls did not touch.

Is the next mark, or finish, to windward?

Next mark is to windward.

As you can see from my background, I'm not a judge or umpire. But in your second scenario, it seems pretty clear to me that Yellow has fouled Blue. Per the diagram, Yellow has sailed six boat lengths and still isn't on a cloud-hauled course to the next mark.

Let's see if others agree.

On the new diagram: A seamanlike rounding onto a close hauled course should have been complete by Pos 4, even if the sails were not properly trimmed for the close hauled course by then. A seamanlike rounding onto a close hauled course for a beat to windward would not include room to drop a spinnaker after the mark. A seamanlike rounding would have the spinnaker on the deck no later than with the mark to windward of the beam of Yellow. Contact at 5 results from breach by Yellow as windward not keeping clear and taking more room than required from Pos 4 onwards. Yellow failed to respond even though there was room to keep clear has Blue turned up, Blue is exonerated for failing to avoid contact based on facts given.

Based on the 2nd diagram and the information now given, for me there is nothing to add to Paul´s statement.
Willii

Al, In your earlier post you stated  

Although, Blue breaks rule 14 because it was reasonably possible for her to avoid contact, she is exonerated under RRS 14(b) because she was the right-of-way boat and the contact did not cause damage or injury.