There is a direct flight on Garuda Indonesia between NRT and DPS (not as bad as it may sound), just in case.

If you fly JL one would have to check if Scoot (TR) and JL do interline meaning if your luggage could be checked through. Else you have to collect it, enter SIN and recheck in with Scoot. With SQ, you should be able to check through your luggage from JL to SQ even if it is on two different tickets / two PNRs, but to be certain that is a question for the JL forum experts as your trip would start with JL and they need to do it.

That is exactly the other question. Do you plan to book this as a single ticket or are you prepared to book two independent tickets on JL and SQ or TR?

Not sure if you want JL because you have status or are earning / using miles or such, but I'm not aware of codeshare flights between JL and SQ for SIN-DPS -SIN so I assume two tickets would be necessary.

Rest is a question of cost and convenience mostly (not sure on your desired class of travel, Scoot is Eco only as budget carrier) and all that.

If you fly into KUL or HKG you have the option to connect to another OW carrier with MH or CX and that should be possible to do on one ticket with codeshare flights and added convenience and protection in case of delays or other IRROPS. I'd go that route as well unless it was vastly more expensive. YMMV.
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