...to the calm lands from Zanarkand ruins? i don't want to waste time trying if you can't, but i need to go back to bribe some 'Chimera Brains' out of Lv4 key spheres. could also do with getting that yoji-whatever aeon too.

so, yeah, can you go back?

(NB, i haven't fought Yunalesca yet)

yes u can once u have the airship

You probably dont need to get the airship to get to the Calm Lands you could probably walk all the way back I dont see what would stop you

1- Walk all the way back
2- Beat Yunalesca and Get the Airship

Don't fight Yunalesca unless your characters are strong enough. If they're not, go back to the Calm Lands and battle monsters there.

Also, get lots of holy water. You're gonna need it in the Yunalesca battle.

...to the calm lands from Zanarkand ruins? i don't want to waste time trying if you can't, but i need to go back to bribe some 'Chimera Brains' out of Lv4 key spheres. could also do with getting that yoji-whatever aeon too.

so, yeah, can you go back?

(NB, i haven't fought Yunalesca yet)
Yes you can. But don't. I suggest that you find a strategy to beat Yunalesca. You are strong enough to beat her if you are ready to change to someone elses sphere grid. I suggest that you build up all your aeons and characters overdrives before the fight, that always helps. Than go to gamefaqs.com and look for a good strategy because I can't offer one that would work for you. Also don't forget about friend and teleport spheres. These will actually work even better at this time of game. I suggest that you switch Tidus to the end of Auron's spheregrid that way he can get a bunch of Strength +4's. Doing all this should make you stronger and make the battle easier.

You really don't need to go to the calm lands yet, you can go later when you get the airship so don't worry about that right now.

Good luck.