To anyone who's been wondering: there will be no need to biopsy the cat's paw. That thing with the definite edges? Scab. Fell off a few days after I posted about it. The next time the cat allowed me to look at her foot, it looked great. I looked yesterday, and couldn't tell there'd been a wound at all. Mind you, I was was holding the cat in mid-air, dangling her feet in front of me, so I only got a few glimpses. But everything looks good!
Yay! Crisis averted.