Skip the Jewel counter and Check in at the MH Counter instead, I believe they are moving from T1 back to T2 on 23 May. Then take the aerotrain or walk back to Jewel.

Try to time it about an hour before a MH flight departs SIN, which isn't too difficult as there is one almost every hour from 6am - 8pm.

They are no OneWorld lounges in T2, and I suspect the MH Lounge won't be open yet either so may want to plan around that.

Quite curious, is it possible to enter T1 airside (or any other terminal) if your ticketed flight is not departing from that terminal ? I know that duty free can only be bought from the terminal the flight is meant to depart from.