You could say "I'll let you" if you're allowing someone to do something. "I'll allow you" is also another way to say that. But it kind of depends on what you are granting.

Not enough context. My guess is it means "I admit that you are right", a rather grudging way of agreeing with somebody.

The sentence is a little strange. I would put it into the future since it's not something confirmed. You can say: I'll allow you (to do) this I'll permit you (to do) this