Forget about this being a finish line.
This is a leeward mark. Therefore 18.1(a) and 18.2(a) Giving Mark-Room applies. All the same as any leeward mark overlap situation.
Mark-Room is defined.

1. Refer Definition : Clear Astern and Clear Ahead; Overlap  One boat is clear astern of another

when her hull and equipment in normal position are behind a line abeam

from the aftermost point of the other boat’s hull and equipment in normal

position. The other boat is clear ahead. They overlap when neither is clear

are sailing more than ninety degrees from the true wind.

Therefore all three boats were overlapped at the Zone as they were sailing more than ninety degrees from the true wind.

2. Rule 18.1 (a)  applies

(a) Rule 18 applies between boats when they are required to leave a mark on the same side and at least one of them is in the zone (Boats were overlapped when A reached the zone at position 1 and entitled to mark room.)

3. 18.2 (a) (1) applies and  Giving Mark-Room (a) When the first of two boats reaches the zone,

(1) if the boats are overlapped, the outside boat at that moment shall give the inside boat mark-room;
In our case B is to give mark room to A; and C is required to give mark room to both A and B.

4. B breaks rule 10 against C; but she will be exonerated under rule 43.1(b) as she is sailing within the mark room entitled to her from C and comply with her obligations to A.

C DSQ under rule 18.2(a)(1) 

My perspective on your questions:
1) yes
2) yes
3) yes
4) yes
5) B would prevail because C didn’t give enough room for A to pass the finish mark per your drawing. A’s starboard side, boom, and/or mainsheet is about to hit the finish mark. 

Question 3: I assume A did not obtain its overlap on B from clear astern as that would be difficult at their point 1 positions. But even if it did it does not really matter.
Rule 11 requires B to keep clear of A therefore B infringes A when she pushes her down from her existing course. B’s defence would be that C failed to give sufficient room for B to meet her rule 11 obligations to A.
(Room and therefore Mark Room include space to comply with obligations of part 2 rules which include R11)
There is an argument that at position 2, B could have and therefore should have gybed which means A would have to as well. That might exonerate C for its later infringement

Agreed with Satish Kumar, we had discussed similar case in our group chat before...