What challenges did expats face when attempting to extend their visas at the Phuket immigration office recently?

Maybe something is in the works to your benefit. In my experience, Phuket is definitely one of the friendlier and more flexible imm offices.

If they turned you away now to wait for a different outcome after Royal Gazette it probably means they know something better that you would maybe qualify for is coming.

I seem to remember that were making people panic by refusing to process extensions just before 2nd amnesty was announced. They could only refuse and say "come back later" as legally they can't announce what is in the works until it's published.

Not saying that there will be another amnesty, i think only an outsider chance of that happening....perhaps just a longer term stay option for tourists to pay for (would be nice) but it is just my personal bias that Phuket is one of the more foreigner friendly and less spiteful offices to work with.

Either way, you seem to have your house in order for the usual embassy extension after the so called announcement that Phuket already advised Multi Non-B people to wait for on 15th, so I would say relax and wait.

Every office and often officer may do it differently go back tomorrow or to another office

Keep in mind an embassy letter is a request to extend a visa. Thai immigration has no obligation to accept the letter and can set their own rules

Why didn't you file an appeal against their decision?