I don't drive either. The way that I get around this is to ask a driving friend for help, or if they're busy, to get a taxi (I've just moved house, so will be registering with a good nearby vet once I've sussed the area, I was with one fairly near to my last address so the taxi didn't cost too much).
But if you're having to scan on a regular basis, it may be worth the cost of a scanner. It might be worth balancing the cost of the scanner against the cost of taxis - the vet won't usually charge for the scan.
I used to live in an area heavily populated by ferals and saw them at all times, roaming round looking for food. Ferals don't usually give their prey as gifts, they need the food too much, so eat everything they catch. Are you sure these are 'unowned' cats? They sound like cats who have a regular supply of food and don't need to eat what they catch. Hope this makes sense.
The RTAs really do need to be taken to a vet for scanning and also disposal of the body. If you prefer burial in your garden, they still need to be scanned properly. It's easy to miss a chip if it's migrated.
I hope you manage to get this sorted before winter sets in. Ferals need good friends.