1. You need to go through transfer security. If somehow connection time goes under the threshold (I can't remember it now probably around 1 hour or a bit less than that) you can use short connection transfer points to clear security. There's only 1 terminal and everything is under the same roof. I'd put 3 hours between flights.

2. My experience with SQ and TK interlining was super smooth. We were flying with SQ on PNH-SIN, then with TK on SIN-IST-CLJ.