I'm almost finished with my first run through the game and I want to make sure I didn't leave anything worth doing out.

1: If I didn't get the summons at the auction house in the WoB can I still go there and get them?

2:is there a way to get Odin & Raiden?

3:If I was stupid and didn't wait for Shadow can I still get him, or am I screwed?

4: Where can I find an Atma weapon?

5: what is the best armor/weapons/recilcs/etc available for:

Edgar
Terra
Celes
Sabin

and is there any special endings??