Hello !
I think this topic may help you somehow : http://forums.tapastic.com/t/anyone-have-convention-experience-to-share/1362
Is thought Bubble a big convention ?
25 copies for each different prints looks really fine to me. Prints are sold more easily than books and offer you a nice price range.
I never sold any buttons, so I can't help you, sorry 
But for keychains, I think the big things that work are popular characters (I got some Zelda and Ace Attorney keychains). If not, keychains don't really work (at least, taht's what I saw on different booths) unless you have a huge fanbase 
If not, try "abstract" element like you said, swords and all those things. Cute things is also a big thing ! You may consider your little bird, then.
Hope it will help you a little 