1. It can do RPGM, Tyrano, Wolf (With the right tools), and a few other VN engines. It cannot do Unity and Unreal. Regardless of the engine, you will need some programming experience when running into roadblocks. My tool is not friendly to people who aren't at least familiar with some Python. It also has a big focus on automated quality MTL's, not so much on editing. Really with the regex module, the engine doesn't matter. What does matter is whether you can directly edit the text through a file.
2. My tool will handle wordwrapping in most cases. If you want to proofread and edit text, I recommend you learn how to edit files directly to do this as it will net the best results. At the very least, VSCode is your best bet and what I personally use to edit and proofread.
3. Since a GPT API token is required yes it will cost money to use their API. On the otherhand, you won't get much better quality wise for an MTL. I have a built in estimator to give you a worst case scenario idea of how much it would cost. If you are worried about mistakes, you can do one file at a time and test to see if everything is correct. This will be more time consuming however.
4. It is the number of tokens according to openai. I recommend you use this to get an idea.
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5. I think at the very least knowing hiragana and katakana goes a long way. Really the more japanese you know the better you can correct any errors GPT throws at you.