If your FC has an airship AND has "found" the Diadem on the airship-map you only need two people to take said airship there (with a maximum amount of 8 per party, obviously).

With two people doing FATEs and the like wont be easy though, but you can just fly around and look for the hidden chests that are all over the place and make your way to the pegasus that way.

To answer the question wether or not the pegasus is the only thing worth grinding for: Well, depends - the namazu earrings are exclusive (at least for the time being, dont know how thats gonna look in 5 years) and if you dont have some of the minions from the old 24-man, specially Calofesteri (which isnt a 100% drop as off now) they might be worth it, specially considering that they're faily cheap.
The beasttribe-mounts are absolutly not worth it, because you want to do the dailies for exp and the quests/storys and other perks anyways.
And the primal mounts are easier farmed by doing the actual fights.