Agree with the rest of your post, but I think I've cracked why that decision was made (if my understanding of the rules is right). It's maybe not as random or illogical as it seems.
Basically - the decision had nothing to do with coronavirus. There is an expectation in Scotland (and throughout association football) that every effort is made to allow games to go ahead even if a clubs senior keepers are all out, whether it's through injury, international duty or suspension. The SPFL expects clubs to find a solution, as does FIFA, and the EFL and the EPL...... If it's outwith the transfer window you find a free agent. It's also within the rules that you can apply to the league for an emergency 7 day loan (that loan isn't available for outfield players). The signings can be made up to 3 hours before a match.
There's plenty of examples of it happening in England and Scotland. The rules were applied a few years ago when Raith applied for postponement and were told to sign a keeper. They chose to stick Ryan Stevenson in goals instead then complained about the decision not to postpone.
So, while Jim Goodwin was saying Scotland was the "laughing stock of world football", the SPFL were simply applying long-established rules.
I'm sure some would argue that finding a keeper with 24 hours notice is unreasonable, but St Mirren managed it so it obviously wasn't that unreasonable.