1) To be honest most corner clamps are just as good as the next. I mean, don't buy a super cheap one, but the super expensive ones aren't all that useful. While metal ones are nice, I like ones with some plastic on the contact point so that the wood isn't damadged. I also prefer ones with two knobs for tightening, not just one. (EDIT: You got me searching, and I got 4 of these for the price of 3 (amazon deal)...and they are good clamps...might want to order 4 yourself)
2) I'm not sure, I'll be going to Home Depot soon to see if they have some (I'm trying to find it too...I'll let you know if they do). Sadly 3/4" is pretty much standard.
3) For speaker building I'm not sure, but Gorilla Glue should work VERY well. For these speakers I'm going to use carpenters glue, with screws, then bondo and paint the whole thing.
Hope that helps, if I didn't make sense anywhere, feel free to ask me to clarify.