You left double food and expected the cats to carefully ration it out over the weekend? Seriously?
Whilst it was kind of you to offer, you should have explained that you were no longer able to do it.
I feed the cats for my mum. You have to do it daily. They do smell, as they wee and poo indoors and it just sits there until someone cleans it. It's going to be worse in hot weather. And it's going to be MUCH worse if left over the weekend, as could reasonably be expected, in the same way a baby's nappies will make a room stink if not thrown out for hours or days. You were unfortunate one soiled on the carpet, but unfortunately that sometimes happens. It's not a nice job particularly, and those who live with animals become desensitized to it all, including the smell. But you can't agree to do it, then at the last moment say that actually you can't do it over the weekend and leave the poor animals hungry, thirsty (cats often tip over water bowls) and with litter trays full of wee and poo. You just can't. And there must be something to clear up litter unless the lady never sweeps up or hoovers.
The key thing is a totally different issue. You're peed off because it's turned out to be an icky job - and I quite agree that it is. She's peed off because you changed what you were willing to do at the last minute, the cats' welfare has been compromised and now (to her eyes) you're just being deliberately awkward. And tbh, it reads like that too. Maybe they should have taken keys but they didn't. Having said that, as long as they can get in the house I don't see a problem with dropping them in on your way to work.
You both sound pretty aggrieved, and both have reason to feel so. But for goodness sake, just tell her it's too far and you can't do it again.