Clearly you’re getting better value from the first 241 and the existing avios, less so from the rest. Would you split up: 1ad and 1ch do an ex-EU and the others use the first 241 with the avios you already have. if both vouchers are in your name, it would have to be your OH doing the ex-EU. To add to the “better value” they’d earn some avios for using with the second voucher, if it’s realistic for you to use it within its validity.

Alternative have a look at any sneaky one-way options back from SIN, and use the avios/vouchers oneway x4?