'I will' is not a complete sentence. You need a complete sentence after a fragment followed by a comma. People can understand it but it is still grammatically incorrect. For example, you can say: If I said I'd be there, then I will be there. If I said I'd come, then I will come. If I said I'd do something, then I will do it.

Yes, they are fine.

No, it does not sound natural. It sounds better in the present i.e. If I say I will be there, I will.