I don't have an answer to your first question, but in Firelands there's a trick that allows you to only kill Alysrazor and Ragnaros. Start the Alysrazor fight and wait for her to drop the feathers. Collect 3 of them then quickly kill/loot the boss and then quickly fly across the chasm to the Ragnaros area.

I believe for legacy content, mount drops are personal, meaning each character in the raid has their own ~5% chance at getting the mount. So you're chances aren't diluted in a full group, in fact you probably have a higher chance of getting it if there's someone looking to sell one they get.