Mays, there are 2-3 other threads talking about this question (how to declare it and not to declare it and the implications when it is done incompletely).  I think 2 were the start line and there was a 3rd where a “do not cross” line was defined in a channel.  

Maybe other forum members can find them and post the links as a comment (I would normally, but can’t right now). 

Re question 2 (and assuming an entitlement to room, as yet not established), B may have been compelled to sail to weather of the pin but A did not compel her to cross the line. B could have remained to weather of the line and sailed around the other end without crossing. So no 43.1(a) exoneration. 

The pin was not a mark of the course for A & B after starting and sailing on the downwind leg of the course. Hence no mark-room.

The ‘restricted’ start/finish line, however, was an obstruction.

Given the said scenario, as inside overlapped on A, B was entitled to room to avoid the pin end of the obstruction under rule 19.2(b).

By sailing through the obstruction B broke the relative SI. Unfortunately a PC might find B was not “compelled” to break a rule if she could have avoided the obstruction by sailing around the other end of the obstruction. Alternatively, B might instead have avoided the obstruction by luffing hard and tacking around the stern of A and around the pin end of the start/finish line. Hence there might be no exoneration under rule 43.1(a).

It’s always prudent to make your intentions very clear by hailing early, loudly and often.

I'm not sure that declaring the start/finish line "restricted" makes it an obstruction under the current definition - "an object, area or line so designated by the sailing instructions."

"So designated", to me, would mean specifically named as an obstruction. "Restricted" isn't defined in RRS so we're left to Mr. Webster. I think the most fitting definition in this instance would be "available to the use of particular groups or specifically excluding others." The line is available to starters and finishers and all other boats are excluded.

Whether that makes the line an obstruction or not is at best ambiguous. It would be better if the SI stated that the line was both restricted and an obstruction. Otherwise, unless the pin itself (an object) met the definition of obstruction I'd lean toward B not being entitled to room. She's obligated to keep clear of A to leeward and to avoid crossing the start/finish line any way she can.

The word ‘restricted’ is not defined in the RRS. It’s common usage does not make mention of anything that relates to a side it must be left on, nor does any definition of the word imply a general prohibition to cross such a thing. Therefore a line described as ‘restricted’ has no meaning, let alone one that could require room.
If an SI uses phrases like ‘…line is restricted…’ it requires rewriting so that it forms a rule.

I don't agree that, on the present state of the SI the line is an obstruction.

There's nothing about an imaginary line on the water that means that a boat could not pass it without changing course substantially.

To prevent just this scenario happening, the race committee needs to expressly designate the line as an obstruction.

Depending on the size of the inflatable tet and the size of the boat, the buoy might be an obstruction.

Here is one older thread:
https://www.racingrulesofsailing.org/posts/368-is-the-start-finish-line-an-obstruction-on-a-downwind-leg

Here is 2nd one I remembered:

https://www.racingrulesofsailing.org/posts/145-closed-line-and-rules-18-and-19

I can’t seem to put my finger on the post about a mid-channel traffic-separation line that boats were not supposed to cross.  That thread had a lot of good ideas and observations in it. 

I think this is the other one Angelo mentioned:

https://www.racingrulesofsailing.org/posts/761-rules-at-a-continuing-obstruction-redux

Thankyou for the links.  Can’t look at them till tomorrow but will fully digest them.