The major primary problem here was that SQ canceled a flight!
Separate ticket or a meeting to attend until late or private reasons whatever where the 8pm flight fitted well so this was booked originally.

In this case SQ canceled a flight by pure economical reasons.
Alternatively 1)canceling the SQ schedule-2)buy a new ticket to SIN at same class and 3)claim compensation of possible additional costs caused by SQ behaviour.
I was a mistake bringing the separate ticket on the table.
Just I cant leave before 7pm and I have to be in SIN at 5 am the next day-fullstop .

Look --what happens when you reserve a table in a fine dining restaurant to invite some friends. You order a nice menue for your friends as well and you pay for it already in advance because the owner asked for it.
2 weeks before the dinner the restaurant owner tells you that they changed the weekly schedule and on this specific day are closed now!
But he tells you that you can come a day earlier- but all your friends took holiday to join or booked a train ticket etc.
So in the first place SQ violated a contract and argued with their T&C but misinterpreted /ignored some facts.

SQ did not everything- they acted selfish!!
-if they cancel a flight they should try to respect the passengers needs so offer rebooking to a flight which departs nearly at the same time. As ususal SQ tried to keep the booking within SQ basket.
So service offred to the customer was not present and the agreement was only reached by with a lot of work/timeload by the passanger.

Best is having the full iternary on 1 ticket its more expensive but does not gives 100% protection(SQ does not like separate tickets at all).

BTW:
In Germany some airlines were taken to court and had to pay compensation for additional costs caused by canceling flights and offering improper rebooking.