My responses to your questions are based on the assumption that you booked an award itinerary in Polaris -

1. You will have access to all the United Clubs in SFO based on your class of travel, but I think the best option will be the UC in the International Concourse - there are reports elsewhere on FT that the front desk staff are sending select passengers to the Int'l First Class Lounge due to overcrowding so you may luck out with that. On your return flight, you will also have access to the Arrivals Lounge which is immediately past customs.

2. The Arrivals Lounge has shower facilities - you will only have access to this lounge as an arriving passenger traveling in Polaris

3. No, unless you also have an Amex Platinum or are willing to pay the day fee to access the Amex Centurion Lounge

4. Your boarding pass for the TPAC segments should be sufficient for entry

Also, on such a long layover, I would suggest heading into San Francisco for a day trip. You have plenty of time to visit Alcatraz or any of the other SF attractions near the BART.