I have a thread where I detailed my preparation and taking of level 5 a while ago, and then level 4 just the other day. I picked up and started studying level 3 materials on that Sunday.
If you grab any kind of preparation book, it will have a little section that shows you common mistakes to avoid and places where they might be strict about the form of kanji. In particular, there are a few times where if you write it like fonts usually show it, it’ll probably get marked wrong, but I don’t know if any of those would apply to level 10 kanji.
For the paper test, I had to wait about a month. But to be fair, they have to check the handwriting of tens of thousands of tests.
For the computer test I took the other day, it comes back in 8 business days (so mid next week). Part of this is probably because there are fewer computer tests spread out over time, and because half of the writing (for any answers that don’t require kanji) is done instead with keyboard input, so it doesn’t need to be checked by a human.