I can’t recall what time SQ start boarding, but in your context (flying J on SQ and wanting to visit Jewel), your main concern should be with the time SQ finishes boarding. I always aim to be at the gate 15-20 mins before departure. At SIN, SQ strictly close the gate 10 mins prior to departure and have never seen them offload passengers for turning up before T-10, although sometimes they do try and hurry people along (which is why I usually aim for T-20). Very different experience to the US!!

One of the things that makes your plan plausible is the lack of central security in T1, 2 and 3. Which means (as I don’t think there are shared gates operating at T2) the only people in your security queue will be on your flight, and IME, security is really quick anytime after 30mins before departure, and reaching earlier than that the queue might be longer but it doesn’t matter.