It could be the mirtazapine. It's not common unless you combine it with something like bup, but it can cause serotonin syndrome, which can present with generalized agitation.
I literally taught my Rosa to understand numbers that way. She was on 5 different meds twice a day at one point, and I didn't use gelcaps for her. I would count down each med, and when I said "one" she would get all excited because she knew treats were coming after that. It's definitely not dangerous, although it can be uncomfortable for you to feel as though you're forcing her for long enough to take more than one pill.
I usually buy from Amazon to be honest. Although I'm sure most pet stores will have the ear wipes, they may not have a good quality probiotic.
Agree. I'd keep the IM appointment and schedule the new vet appointment later if you still feel you need a regular vet appointment right now.
If not, maybe give her a few days/couple of weeks to decompress from the IM appointment and then make an appointment for a new patient exam just so you're not having to go over the basics if you need a vet more urgently later on.
Personally, I would not put a liquid in a gelcap. That's going to get really messy, really fast. And if the liquid makes it through the gelcap while it's still in her mouth (likely at some point), it may put her off the whole idea of gelcaps and make pilling her other meds more difficult too.