1: If I didn't get the summons at the auction house in the WoB can I still go there and get them?
No. Golem and Zoneseek are only sold in the WoB.

2:is there a way to get Odin & Raiden?
No. Odin changes to Raiden, so there's no way to get more than one.

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

4: Where can I find an Atma weapon?
In the Cave to the Sealed Gate (WoB), somewhat near the end of it, there's a platform with switches on it. Press the correct switch, and you'll hear a distant sound, which is a door opening. Through the door is a tiny room with the Atma Weapon and three pieces of Magicite--that's the use-as-item-to-call-random-esper magicite, not magicite you can equip and learn from.

I have also heard that if you don't get that Atma Weapon, you can get a Dirk and bet it at the Coliseum for a slightly more powerful weapon, then keep betting up, and eventually you'll get the Atma Weapon. I've always gotten the Atma Weapon in the WoB, though, so this has never worked for me, and I have never tried it without already having an Atma Weapon.

5: what is the best armor/weapons/recilcs/etc available for:
Edgar Terra Celes Sabin
Genji stuff is always great. Sabin's best weapon is the Tiger Fangs, and Terra and Celes should use the Atma Weapon and the Illumina, if you have it (I think the Ragnarok esper's a better deal, but if you want the Illumina, have the weaponsmith make it into a sword and bet it at the coliseum). If I were you, I would see if I could find a decent coliseum list on the net (or can you get to the one in the game before you finish it?) and see what I could bet to get better weapons/armor/etc. I would definitely find two Snow Mufflers for Mog and Gau, though, and see if I could get two Red Jackets for Sabin and Edgar.

And there's no special endings, but if you don't get all 14 characters, some scenes won't be shown.