I don't know who he is. Bob is a very common name. Must be from the US of A if the links are any clue. And he's a judge doing rule 42 stuff on the water.
Anyway, a new rules blog with a posting about rule 18 and markroom: Inside, Outside, Upside Down. Go and have a look.
My take on the rules-question:
In position 1 Yellow not only has to give mark-room, but is also keep clear boat. (RRS 18.2(b) & 10)
Yellow is not keeping clear in position 2, particularly if Blue has to luff to avoid her. (RRS 10)
Although the overlap was broken and a new overlap established, in position 3 Yellow still has to give mark-room AND keep clear, now under rule 11. (RRS 11 and 18.2(c))
And finally in position 4 Yellow is not entitled to mark-room, she's not keeping clear and she's preventing Blue sailing her proper course round the mark. (RRS 18.2(b), 11 & definition of Mark-room)
I do wish Bob all the best with his new blog. I've placed a link on my list.
I've just tried to give Bob a heads up about this post by trying to comment on his post; seems to be restricted to team-members only.....