If Scully was someone's pet, it shouldn't take too long for him/her (I'll go with her) to start trusting you more. As long as you don't try to approach her, she should get more and more used to you and come closer on her own.
The ear tip, though, would indicate that she's been fixed through a TNR program - maybe mistaken for a feral. It's pretty unlikely that an injury would be a clean cut. Do you know if the feral colony is managed by a TNR group (do any of the cats have tipped ears)?
If there's no shed or garage, do you have a place that's somewhat enclosed or shielded from the rain, like a deck, where you could put a little shelter?
If you do a search for cat shelters on this forum, you'll turn up some really good DIY ideas, photos, and links to useful how-to videos. A plastic storage container turned on its side, with an opening cut out on the lid works. You could fill it with straw, which is best for outdoors. Not too big, because small spaces keep heat in. Putting mylar sheeting on the inside also reflects body heat. If you can elevate it a little so it doesn't get wet, that's a plus too.
If there's an outdoor outlet, and you're willing to spend about $35, you could get a heated pad designed for outdoor use to put inside the container. I've had one for years, and it uses very little energy, even when plugged in 24/7: Heated Cat Beds: Indoor/Outdoor Heated Pad for Cats
So when Scully comes for food, she's not followed by the other ferals, right? If that's the case, I'd slowly move her feeding spot to where you'd have the shelter for her. I don't think the other ferals would come that close to your house.
Snap a pic if you can - we'd love to see her!
In order to keep Scully coming to the shelter but keep the other kitties away,
If this kitty is coming up to your front door but the other ferals aren't,