1. Not understanding your question. I-539 is to change to B2.
2. You can submit upto 60 days after losing your TN, or when your I-94 expires, whichever is soonest. Once you apply for B2, there is no more TN grace period. You are now in B2 status for the period you requested on your I539.
3. As is always stated here, as soon as you get a new work status, you MUST formally cancel any other pending applications, otherwise if one is approved (or denied) later, that will become your new situation.