US Sailing Appeal 2 is worth reading. The facts are slightly different as it involves rule 18 and rule 20 but it has a similar conflict between two rules.
Following are diagrams for the four scenarios John C outlined. When a boat goes between the mark and the obstruction they have denied the other boat the room to go there too. The exception being if they hit the mark or obstruction. In every scenario I can think of where one or both boats sail between the mark and the obstruction at least one breaks a rule. If both boats round the tree and the mark neither breaks a rule.
Scenario 1 – The original post. Sleigh Ride takes mark-room and denies Polar Express room at the obstruction.
Sleigh Ride does not give Polar Express room between her and the obstruction and breaks RRS 19.2(b).
Scenario 1
Scenario 2 - Polar Express and Sleigh Ride go between the mark and obstruction, Sleigh Ride hits the mark, no other contact.
Polar Express outside overlapped at the zone failed to give Sleigh Ride mark-room, and broke RRS 18.2(a)(1).
Scenario 2
Scenario 3 - Sleigh Ride goes to leeward of the mark, no contact between any boat or object; Polar Express goes between the mark and obstruction.
Polar Express breaking RRS 18.2(a)(1).
Polar Express is not exonerated for breaking RRS 18.2(a)(1) as it is not in the list of rules she gets exonerated for in 43.1(b).
Scenario 3
Scenario 4 - Sleigh Ride and Polar Express both go between the mark and the obstruction, not making contact with either but making contact with each other.
Sleigh Ride the outside boat at the obstruction, failed to give Polar Express room to pass between her and the obstruction, despite being able to do so from the time the overlap began. Sleigh Ride broke RRS 19.2(b).
Polar Express outside overlapped at the zone failed to give Sleigh Ride mark-room, and broke RRS 18.2(a)(1).
Scenario 5 - Polar Express and Sleigh Ride go between the mark and obstruction, Polar Express hits the obstruction, no other contact.
Sleigh Ride the outside boat at the obstruction, failed to give Polar Express room to pass between her and the obstruction, despite being able to do so from the time the overlap began. Sleigh Ride broke RRS 19.2(b)
Scenario 6 - Polar Express and Sleigh Ride go between the mark and obstruction, Polar Express hits the obstruction, Sleigh Ride hits the mark, no other contact.
Polar Express outside overlapped at the zone failed to give Sleigh Ride mark-room, and broke RRS 18.2(a)(1).
Polar Express to windward failed to keep clear of Sleigh Ride to leeward, and broke RRS 11.
Sleigh Ride the outside boat at the obstruction, failed to give Polar Express room to pass between her and the obstruction, despite being able to do so from the time the overlap began. Sleigh Ride broke RRS 19.2(b)
Sleigh Ride caused contact between Polar Express and the Christmas Tree even though it was reasonably possible to avoid causing it, and broke RRS 14(c).
Yes David, in that scenario would be for sure. I just use the rough 50-50 average here to mentor the young lads (ie don't hide behind it or throw in the towel just becasue of one's handicap) - and inside the boat 50% is the boat itself + 'technology developments' (getting a flying jib or something or rather) by owner, skipper and crew, to gain advantge over competitiors (handicap affect gets a lot less)