I'm not going to weigh in
on the sensitivity of the seat occupancy sensor. I'll just offer a small point regarding the pet-warmer application:
Starting this past fall I semi-adopted a stray cat that was hanging out around my property, and was injured in some kind of altercation, probably with a bobcat or coyote. He's not tame enough to approach me or come inside, but he gladly takes the food and drink, and the nights get somewhat cold in the winter. So I put out a couple of heating pads for him to sleep on.
When I was researching the products to buy, I noted that multiple sources said that warming pads for pets need to be milder then what humans tend to like. The implication is that it's very easy to overheat your pet with a warmer designed for humans. I'm not saying it's dangerous, but maybe more uncomfortable than you'd think.
So just a thought, you might want to ensure that your puppy's seat warmer is on on the lowest setting, and perhaps consider a folded blanket or something like that to keep it cozy but not too hot.
I hope you're also taking measures to mitigate what could happen to your puppy if she gets thrown forward in a hard-braking or collision event. But that's an obvious issue; the pet heating-pad issue is one that I hadn't known about until recently.