When flight attendants are rude or even hostile, my approach is to gently ask them if there is a problem. Call them out (nicely). Let them know that you know that they're not performing up to expectations. Do this preferably within earshot of a witness so you don't get accused of disregarding a flight crew's instructions or worse.
Barring that, things are not likely to get any *more* uncomfortable, and maybe the gentle goading will help them realize their behavior is not being perceived well.

Of course if service is just indifferent, it's harder to address directly IMHO.

I guess the question for the OP is whether the real issue was the FA behavior, or primarily the car seat. Didn't hear many specifics on the behavior issue in the OP. I've been skipped over for PDBs, usually it's not intentional, and I'll just flag someone down and ask for a drink. And I would think once you showed boarding passes for business class, the greeter would have let you through.