Grief, the plot thickens. Where on earth did the TG flight come from? I'll reiterate what I always say - FT is at its best in giving advice when presented with full and accurate scenarios. This is about as far removed from that as I can imagine.

Anyway, I assume you're going to try and sue either SQ or the Travel Agent in the US.

I'm not familar with US law/DOT guidance around flight changes, but certainly if I take one rather large US airline (I'm looking at you, AA) a schedule change of that magnitude wouldn't even make me blink, so assuming they act within DOT guidance, and you got a bit of notice (and your words suggest you did) then it's exceptionally unlikely any court in the world would side with you.

If you try and sue for the replacement flight cost, particularly given you've admitted you neither accepted nor rejected the change (i.e. you didnt give SQ the opportunity to offer you something different) I'd put your chances of winning as slim. If you took that action in the UK, I'd expect the judge to regard your claim as frivolous and award costs against you.

What you do have, given there was a flight cancellation involved, is the absolute right to a refund of that sector under IATA rules or suitable flight change. Given the cancellation, If you'd proposed a flight change to SQ instead of asking under their bereavement policy, they'd very likely have done it (and the TA should have been able to arrange that quickly). You could still have that, provided any new flight fell within IATA ticketing rules, but as you never want to fly with SQ again, your sole option is now refund. SQ has offered you that.