1. You can check SQ website for availability for the individual segment to SEA. If available, call SQ to book a *A award that includes for MW-SEA-SIN-INDIA as one-way *A award. The KF required is by zone, should be NORTH AMERICA to CENTRAL, SOUTH ASIA.
*A redemption chart before 5 July: https://www.singaporeair.com/saar5/p...ipAwds2021.pdf
*A redemption chart on and after 5 July: https://www.singaporeair.com/saar5/p...ndTripAwds.pdf

Some agents may attempt to charge you a separate KF for the US domestic segment, especially if it's not in the same class as the rest of the journey, say MW-SEA in economy and the rest in business. If so, insist that they price it end-to-end as business class, even if you're travelling the US domestic segment in economy.

2. Impossible to tell. Depends on route popularity, travel period, travel class, etc. Revenue management is a black box mystery.

3. No, waitlisting is only for SQ flights.