The first issue...

Number of first class seats has no relationship to if you can use the PMU. There must be seats in that "category" open. ExpertFlyer.com will help you figure it out. (There's a free trial so you can see how it works If you fly a lot I recommend it.

As for 1. It's up to you.

#2 Probably not. The PMU avaialble seats go fast and generally there aren't many open that late.

3. Folks say you can. I have never tried it.

4. Plan to sit in coach. It might happen, but my international travel on Delta has been in the cabin I booked about 95 to 98% of the time. (2 OP UPs out of a LOT of flights, one to business and one business to First back in the old days )