
Payne Mykonos |

From text " While fused with his eidolon, the synthesist can use all of his own abilities and gear, except for his armor."
My GM is unsure how me using potions (or read scrolls or wands) would work without hands per-se.
The Eidolon is basically a dragon with claws, not hands. And can he/I/we reach into my backpack and retrieve any gear?
Does amulet of natural armor stack? Does a ring of protection? If armor does not work, do I lose my arcane spell failure?

Fuzzy-Wuzzy |

Amulets of natural armor give an enhancement bonus to natural armor. This stacks with any actual natural armor bonus you have, but not with other enhancement bonuses to natural armor.
Rings of protection give a deflection bonus. These stack with anything except another deflection bonus.
If armor does not work, do I lose my arcane spell failure?
This has never been explicitly addressed AFAICT. However, an Ultimate Magic FAQ says
Summoner: Does a synthesist (page 80) keep any armor bonuses or other benefits from his armor when he is fused with his eidolon?
No. The eidolon is, in effect, the synthesist's armor, and any armor the synthesist wears does not function while the eidolon is present. Fortunately, this doesn't mean the synthesist has to remove his armor when wearing the eidolon, and when the eidolon is banished/dismissed/killed, the summoner's worn armor immediately begins functioning again.
Personally I'd read this to say that since eidolon effectively is your armor your ASF from any real armor goes away, but it's certainly arguable. Check with your GM.

I am Nemesis |
Do you have arms? Then you have suitable hands for hand activities (it doesn’t matter on a rules basis here, but dragons can use wands and potions and scrolls). Does amulet stack with what? It works as normal.
there's a dirty joke in there i'm too innocent to finish.
amulet of natural armor stacks w/ eidolon natural armor.
ring of protection is a deflection bonus that does not stack with other deflection bonuses, but you do get the higher of the two.