Reporting back, on a separate ticket transfer in SIN, AY to MH.
Pre HEL departure I had done online check-in in the MH app, with check-in procedure going through all the way, with "warning" that online BP could not be produced but to be picked up in SIN.
Arriving in SIN on AY131 ≈20 minutes late, like 18:40, at T1 gate D40-ish. D-pier transfer desk is literally a 2 minute walk, but alas it was closed and C-pier transfer was suggested. The trek from D transfer to C transfer is maybe 5-8 minutes in leisure tempo. Desk was well manned and line was short. <5 minutes wait.
Agent asked for incoming flight details and bag tag (had none as I was carry-on only) and did not want to see SG arrival card. As i was on a one way MH ticket to KUL, I was also asked my stay duration and if I had return flight arrangements. Some tapping and boarding pass produced, with QR platinum status visible, for gate D37.
Clocked in to Qantas first lounge (basically in the D-pier) at 19:00, ie the total time to get onwards BP including having to walk across entire T1 was 20 minutes. I would have made the 19:40 departure, despite AY late arrival and C-pier trek. *) But who is in a rsuh when there is QF first...
*) AY131 can be 1h late too, so the 19.30-ish connections that Finnair sells are still risky