Hi lemontie13, welcome to FT.
Yep, they will tell you where to send your boarding pass stubs. Sometimes you can get away with fax/scan but the official rule is to send original boading pass.
Singapore, Continental, Air China and Etihad are all 6 months. The rest are 1 year for retro claim.
This is correct, although Continental no longer exists. Yes Asiana need to update their website.
As for getting from SEA-NRT, that flight is 4769 miles so a round-trip comes out as 9538 miles which will cost you 55k. You can take either United or ANA depending on whether there is award availability on either. The codeshare restriction just means that you can't book an award on ANA with a UA flight # or vice versa.