i guess the issue here = you can purchase a single itinerary with SQ + Scoot (b/c SQ doesnt serve your final destination), and no lounge access when transiting SIN

LH/LX have Edelweiss, similar problem (but LH/LX have carve outs for their own elites + J but not *G). and technically its still based on your departure flight, nots ure if you can get a long-haul J on LH connecting to Y on Edelweiss


UA has UnitedExpress but its treated the same as mainline UA (in terms of lounge access / mileage accural)

NH has AirJapan(NQ) which flies shorter routes like SIN-NRT but its treated same as UnitedExpress above.
however,they're introducing AIrJapan (proper budget airline), so unsre what happens. NH does have Peach but dont think you can buy a single ticket with both NH + Peach?