Seeing as we're now home & safe, this may help someone.

After the dreadful experiences with SQ customer service I listed above, you may like to know I was lucky enough to make contact with SQ Customer Relations via email.

I wrote "URGENT" plus my PNR in the email title & sent it to:
[email protected]

Someone named Nitivadee R. responded after around 30 mins & continued to support us during the next 14 days taking no longer than 2 hours to respond each time.
This included keeping the return tickets open, clarifying SQ requirements & rules wrt flying after Covid & arranging & issuing the return flights & tickets.

I cannot praise Nitivadee enough. Outstanding indivudual without whom life would have been extremely difficult indeed.
When we needed them most Singapore airlines were generally dreadful, Nitivadee was a brilliant shining light, thank goodness.