Hello anon!
I’d like to break your message down, since there are quite a few ideas in there and you seem a little confused. (Disclaimer: it’s a complicated topic, and I am not The Ultimate Authority on Jewish belief. With that said):

I know ZERO things about Judaism and being Jewish (I’m a deist kinda)

There are many great places online to learn more if you’d like!

But is Judaism like agnosticism?

Judaism is a religion, but the Jewish people are also an ethnoreligious group. Not all Jews are religious or believe in G@d, and not all Jewish denominations require that you believe in G@d. However, for the purpose of this answer, I’m going to speak from my perspective, as an Orthodox Jew who does believe in G@d.

You say in judaism one can “wrestle with God and challenge him on things.” But if one doesn’t like what God does and thinks he does/says dumb things, why bother believing or worshipping them? Wouldn’t one be “better off?”

The way you worded this question is interesting. Rather than nitpick ( for example, wrestling with G-d doesn’t necessarily mean you think G-d does/says dumb things), I’m going to assume that the general question you’re trying to ask is: why would you believe in a G-d that you sometimes disagree with?

And the simple answer is: why would you believe in a G-d who you don’t sometimes disagree with? If all your beliefs match perfectly with your god’s, and everything lines up exactly between your own life and your belief system, you don’t have a g-d, you have a sock puppet rubber stamp-ifying your opinions. (Credit to @animatedamerican for inspiring that image, and for general input into this post.)

Ultimately, the world is a confusing place, and no belief system of any sort is ever going to make complete sense to anyone, whether from a logical place or from an emotional place. (I would, in fact, argue that not struggling with your belief system is indicative not of the inherent amazingness of your belief system, but rather of a lack of critical thinking on your part.) As a result, religion, and ultimately G-d, don’t have to make complete sense to you — there can be parts that you wrestle with, and despite those parts, your religion can still be the most fulfilling perspective on the world for you.

In addition, you can be religious without a complete understanding of G-d, much in the same way you can form relationships with other people without having a complete understanding of all human psychology. From a personal perspective, the perspective of a religious person, I know that G-d is beyond human comprehension, and as a natural consequence of that, I’m not going to understand everything that I believe G-d is telling me to do. In fact, a large part of my religious experience is taking the questions I have and trying to understand how to answer them, fitting the worldview I have from experience and from exposure to various philosophies into a Jewish understanding of G-d — not only that, but a large part of my religious experience is knowing that I will never in my lifetime understand G-d.  Besides, I know that I can never fully understand the natural world, and that means that if there appears to be a contradiction between something divine and something that I observe, it must be that I’m missing a piece of the puzzle, either from the natural side or in my understanding of the divine. The fact that my knowledge isn’t infinite doesn’t stop me from believing in G@d.

If anyone has anything to add, as usual, please do! You can ask me here, or reblog and add, or reply, or anything really!

I wish I did! The problem is, we do have folklore, but the major accepted texts are practical; how to deal with everyday life. So what I do know I usually know from extrapolating from the base of Jewish law knowledge that I’ve accumulated over my life, and by asking jumblr friends or knowledgeable relatives. 

You could look into Rabbi Slifkin’s books and blog - I believe you can find him here.

One thing I would warn about is that a lot of the original mystical/supernatural areas of Judaism require a large amount of background knowledge. Midrash, for example, often assumes a working knowledge of all of Tanach (that is, Torah, Nevi’im, and Ketuvim). Similarly, if you’re looking at aggadata from the Talmud, you have to have a large knowledge base to understand them properly in context.

If any of my followers have suggestions, please add them on! I’d love to know!

1. Would the changeling require a formal conversion to be Jewish?

It’s kind of hard to say! The real question you need to ask with a changeling is not whether “would a changeling have to formally convert in this specific circumstance?” but more, “wait, can a non-human even convert in the first place?” (That’s the question I asked @bigsis144 when I first got this ask, so credit for a lot of this answer goes to her.)

The difficulty is that there aren’t examples of creatures that are both non-human and Jewish, or of creatures that are non-human and try to become Jewish, in texts I’m familiar with. (Come to think of it, the Werewolf Binyamin person does seem to have a non-human Jew — explicitly stating that werewolves were created during the first six days of creation — and some later commentators say things that might agree that there are non-human Jews, but I kind of have no idea what to do with that information, so I’m just like, pretending it doesn’t exist? I’m a good person, I promise. Anyway, not in any early or straightforward texts.)

To be Jewish, though, something has to have free will. Angels, for example, are considered real in Jewish tradition, but they couldn’t be Jewish because they have no free will. I guess it kind of depends on what your changeling is like. If it’s fully sentient, it’s plausible that the changeling could have enough free will to technically be eligible to be Jewish. If it is eligible, then it would need a formal conversion — it isn’t born to Jewish parents, let alone a Jewish mother.

2. Would the changeling count under pikuach nefesh?

I’m not entirely sure what you’re asking. If you’re asking whether the changeling could be in a situation covered by pikuach nefesh (the principle that saving a human life comes before other commandments) — for example, if the changeling hurt itself (themselves?) on Shabbat and needed to be rushed to the hospital, could you break Shabbat to save it? — I would feel pretty strange saying it’s human enough to be Jewish, but not enough to qualify under pikuach nefesh.

If you’re assuming that the changeling aims to kill you, then it wouldn’t be pikuach nefesh that’s relevant but rather the principle of rodef. A rodef (literally something like “pursuer) intends to kill someone, and you are permitted to kill them to prevent this. In modern life, I’ve most frequently heard it brought up to discuss abortion of a fetus who is endangering the life of the mother; in this case, as far as I know, the fetus is considered a rodef and can be aborted to save the mother. In this case, the changeling is clearly a rodef, and if lesser methods cannot be used, it would be permissible to kill the changeling child.

Hope that answers your question! If anyone has input or anything to ask, you can come to my inbox, message me, whatever!