For question 2.

According to the instructions given to referees by certain journals ... the submitted material is to be treated as privileged information. And (one consequence) you cannot use it as a basis for your own further research. Of course, once the paper is published, or the author otherwise releases it as a preprint, then of course you can use the information. And in particular you could then contact the author about a collaboration. (You need not mention that you were a referee.)

So I reiterate one of the other responses: do not do it without the approval of the editor who sent you the paper for review.