Potential Newbie to Ambassador SFC-help please!!

[pilot]

OK-UK guy here, off to the US in sept.

Already a plat Member- more of an HIX/HI/CP kinda guy.
Looked at rates for 20 sept + 21st at SF in town-not the airport.

Best I can see realistically with brekkie is:
IC SF first nite-$250 with brekkie
2nd nite-$650 with brekkie!!

Both nites booked seperately/flexible rate

If I apply for ambassador now- will I get the voucher in time (6 weeks) and can I just pay for the first nite and flash the voucher for the 2nd nite on checkin and so pay just the $250??? Or will they average out the rates?, so I end up paying $450 odd. Will I get brekkie both nites?

Many thanks for your advice/experiences!