Trying to reconcile some Jewish vs Christian… ish customs

So, I want to just preface with this, my girlfriend, who I fully intend to make my fiancé and marry within due course, is Jewish. While their grandparents were survivors of the war, and some of their cousins are significantly more orthodox, their household really isn’t.

My parents were more or less. Christian. We learned much of the same stuff, my mother was even fervent enough to make me copy the book for punitive measures starting from the Old Testament (I know the term is bad, old inaccurate, and stuffy, but I never scribed past that) but otherwise flitted between denominations and grew less stringent as time went on. With the younger children getting worse and worse behavior wise, but that’s not really the point here. I’ve long since cut ties with them with the exceptions of helping when they need, ask, or anything regarding the younger ones.

We’re planning to go over for Passover this year, I was invited last year as well, as their father whose parents were German by birth and himself primarily protestant didn’t specifically wear any ceremonial accessories, I didn’t feel as though I needed to. Over the course of the year since, it’s been clear that some customs do hold greater meanings and some don’t for them. So I’m unsure if it’s best to offer a bottle of certified kosher for Passover wine again, or wear anything specific (They did collectively love the kosher for Passover wine I provided, but I worry I tainted the purity of it by purchasing it {no, they’re not that strict, but I overthink regarding anything with her} ultimately it was a semi sweet Moscato.)

We’ve talked about several of these scenarios, and generally I ask for her advice on what should be done in specific situations, but now we’ve gotten to more serious topics. The topic of weddings has come up. I don’t have many suits, what I do have is generally reserved for weddings and funerals. They’ve expressed that their cousin’s wedding that they want to bring me along to is more traditional. Which instantly raises many questions. Is black okay? (They’ve said it is) is their customs/events I should be concerned about (bouquet catching, pack my jacket pocket full of candies [Aufruf] I normally have peppermints and tissues to help guests/family that may have sniffles/teary eyes should that be something else?, any particular hymns/lyrics I need to memorize.

Researching traditions also calls to mind several that I worry I won’t be able to fulfil on my end either, or are just, uncertain of. My mother’s since passed away, so both leading me to altar is out of the question, my father and I also are on disparate fronts on many other issues so that may be hard to convince, is mezinke required? how do we specify that she’s the youngest for that? and for both of these… situations would it be wrong for me to wear a yarmulke, or rather where would wearing one be respectful?

As an aside, while they say they’re not particularly religious, it’s clear some of these holidays hold special meaning to them, and while from a Christian background (more agnostic) we learned many of the same messages, we didn’t learn how to celebrate, is there a place I can learn more?

I get it, I have lots and lots of questions, probably more than I should expect online, but I really do appreciate any feedback, or at least the answer to… where would be a proper place to direct these questions without her specifically being the target of said questions. (It’s much easier to ask “Do you want me to do x thing than ‘let me know what I should do’)