hi rsb
i wud say
1) wall first as u dont wanna keep walking over tiles while working an poss ruining them in fitting time.
2) I would reconmend priming the walls an floor before as well as using a product such as Mapegum from Mapei check out there website for all products
MAPEI adhesives sealants chemical products for construction all tile giants stock their products.
3) not 100% sure but i sudn't think you would need one.
4) Id say both.
5) many people have their own preferences once again id say check Mapei for their products.
6) back buttereing is essential on all floor and wall to fill in all the air pockets making a stronger tile and giving you a better grip and solid tile.
7) any travertine tile needs to be a powdered white flexiable adhesive, white because of the pourusness of the tile you dont want it soaking through grey adhesive and spoiling the finish. again look up Mapei id reconmend the keraquick or keramaxi which is an S1 adhesive. Keraquick being quick set Keramaxi being slow set.
8) some tilers swear only by slow set adhesive they say it gives you a better bond to the tile cos it has more time to adhere to the tile an wall better look for Keramaxi again (I know i repeat myself alot but i think Mapei is the best stuff.)
9) cant really help on the step up but i can tell you something u dont want to here an that is for a travertine tile i wud be reconmending a much bigger bed than 6mm adhesive, more like a 10mm minimum sorry.
and last of all please dont forget about the sealing process, the company
www.LTP.co.uk have everything u need, start with Grimex to prep ur area, Mattstone to seal and Waxwash for ur after care best stuff to keep ur tiles looking brill.
hope that is some help, but take all my word for it have a good ask about an hopefully people will back me up on my understanding.
Regards
Steve