what's the usage of migration? why AI provide blank treaty? What's the treaty of alliance?

One thing I can tell you is that taking Alien Migrants allows you to shuffle populations of them to planets that would normally have to be domed. So that Domed Huge planet that was only giving you 5 facilities suddenly becomes worth 25. That alone is worth the migration. Just accept a couple of turns from them and then change the treaty, and shuffle the proper populations to get those Domes torn down.

We shall then, make the rubble bounce ~~~ Winston Churchill

Immigrants only come to planets they can breathe. If they come to a domed planet, that means you need to jettison/scrap your own people (or transport them off) for the good of the planet. Don't worry, they're loyal citizens and so there's no difference between them and your people (aside from whether they're Emotionless or not).

---Sig---
Playing Touhou games (Go here if interested . No, nobody else is that good/insane as that replay). No rush for SE V bug fixing.

I'd like to know the answer to #3. I've only been in one alliance to date, but I played till the end with being in it.

alliances are still bugged and normaly only make problems. so best for now is you just don´t use alliances in AI games.

Furthermore, I've seen one blank treaty in all my time. Blank clauses in treaties I've seen yes, but not a treaty with absolutely no clauses.

---Sig---
Playing Touhou games (Go here if interested . No, nobody else is that good/insane as that replay). No rush for SE V bug fixing.

I second Psieye and add that I have never seen a treaty with no clauses in it at all, but I have also never seen a treaty with somthing from all the posible clauses in it either.

I suspect that if you saw one its due to a unlikly and dificult to reproduce sequence of events or a player in a multiplayer trying to mess with your head. I would add I am not sure if its posible as a player to create a clauseless treaty either.

I would check but the 286 laptop I am using wont even run SEIV and I must be the last person alive using netscape 4

No blank treaty. There are treaties with clauses but without clause parameters where necessary. Normally the format is like:

Title | Share research
Para | the XXX Empire share its tech...

It is common that AI "forget" to click the checkboxes. It's a bug however it doesn't matter. You can just ignore that article. A CLAUSE WITHOUT PARAMETER WILL NOT TAKE EFFECT.

Also there are blank requests. Your ally beg you for something which is in fact nothing. I suggest you just accept it huh huh! Of course you shall not transfer any resource. And maybe the relation between you and your ally will warm up? God knows...

So do alliances work for multiplayer?

Though those non-specific treaty clauses pop up virtually every time I'm offered one under the 1.33 patch, I've yet to accept one, so it may have changed.

I'm frequently pestered for handouts. Accepting does nothing. It's hard to judge whether responding with a "give gift" does anything, since their response & attitude changes are identical to what happens if you gift an AI unprovoked.
So far, the only kind of demand I'm sure can be met, is the "Lend us spies" thing.

AI allies will always ask for "economic aid". But never have I seen anything in their wishlists(like certain amount of resource, techs...). They say thank you when I agree to help however no real material transfer ever happens.

I haven't played for long. Features mentioned are based on 1.20 patch.