For those who kindly advised me, here's the pros and cons.

I'd have to take kitty 150 km. to find a vet who won't charge me arms plus legs for a whole day quarantining and cleaning their surgery, due to fears about feral diseases. SPCA here only offers euthanising, really. I haven't given up yet, but am not lookin' forward to hauling a wild cat that far and back.

'She' turned out to be a 'he'. I'd previously looked for the usual male appendages at the back of the cat and didn't find any, but the cat has thick long fur. Then a couple weeks ago I caught kitty licking his unmistakable penis. I think the cat is younger than I previously thought, since it's grown to twice original size since I started feeding it, and has not started spraying, AFAICT.

Progress: I can pick this cat up, pet and cuddle it, call it and it will come. Nobody else can get near it, unfortunately, because I want him calm enough to take him to a vet. He let me cut mats out of his coat with scissors, put flea stuff on his neck, will eat from my hand, has never shown aggression towards me, just skittishness if I move too fast.

Survival: I gave up and made him a warm little house on the deck in time for TS Noel, from which he can see in through the patio doors. He likes looking at us, and will come in for brief visits if I'm alone. Our own cat thinks he should keep the heck outside, though, and inevitably starts beating on the stranger, who is half his size.

Hope: I dunno... I've decided to just try to get him tamer for a while before going any further. My own cat's up to date with needles, and this fella appears to be perfectly healthy, although i know you can't always tell.