Answers to your questions:
1. ILR is the one you should apply for
2. Correct ... you can apply 28 days before your 2 years' residence is up
3. It's really a matter of personal choice ...... if you apply by post you could find yourself waiting anything up to 3 months.
Applying directly through a PEO means you will be given the result on the same day. But, of course, this privilege will cost you £300 extra.
Goodluck with whatever you decide.
