Sorry, you are mixing a lot of things here, so I'd suggest to read up on current AMB program and make sure you are clear what it covers and what not (it changed a bit post Covld). For starters, the AMB really only matters at ICs as you know and here you'll get 10k points or 50USD credit (F&B) per stay if they can't upgrade you. You move into the realm of Indigo, CP, HI etc. and only your Diamond matters and it doesn't guarantee those things.

For the FWNC, it does no longer work at Kimpton or Regent, that was only during Covid. Now it is purely at ICs so there is only one IC Grand Stamford and that place can be very busy from my recollection so certificate usage means usually come early though last minute has been possible pre-Covid.

For your SIN example, there are two ICs to pick and you should never book your entire stay at AMB FWNC rate as you are right that it will be Best Flex at minimum or even slightly inflated rate above Best Flex. It is good that they offer the rate outside of weekends, but I'd try to reconfirm with the hotel that this is intentional and will be honored. This was possible at Kimptons and Regents IIRC during Covid, but is not common at ICs.

For your planned 3 nights, just book the minimum two nights on the certificate rate and the extra night separately and likely cheaper. No issue to keep same room usually as long as you book same room category and of course you can email the hotel prior to your stay to link the reservations and ask to assign the same room.