I also leave the wet food out for hours and Cosette takes her time to eat it. Knowing you're gone may be just the thing she needs to eat. I've done it for months and we've never seen signs of sickness from it. I'd try that before giving too much kibble.
Yay for success! Whatever works! The less disruptive the better.
As for your questions...
1. Doing it once shouldn't cause major damage. In the future be careful not to get her in the box every time.
2. She'll learn the routine of ointment once a day, and when she sees it's never at the food...
If she's still coming to you for scratches, it means she's not mad at you. She's willing to forgive you after a negative encounter.
Honestly I think you're trying to too hard to make the ointment a pleasant experience, and you run the risk of her associating snacks with ointment. Don't...
Sounds like the female has other resources for extra food. I'd stop putting food out completely. Even if the neighbors try to reign them in, cats gonna cat, especially if the cats are accustomed to roaming.
Cosette is definitely feeling better. She still stays under the couch, but she's much more receptive to attention, and she's more playful when she comes out. Thursday she goes for checkup. Getting plenty of food into her before the pouting on Thursday and Friday!