I took SQ for the first time, on SQ22/21 in Dec. 2 US *A award tix. Superb airline. Anyways,

1. Correct. You can put in requests at time of booking. They honored mine.
2. Yes. Though instead of 48hrs out, it was more like 47hrs out that I could log in. Don't panic, but do get the SQ locator number for your reservation (not the US one).
3. I will only say SQ demonstrated nothing but class, organization, and efficiency in processing even my seat re-request (I changed my mind) at the EWR counter.