1)Yep, as free time project it may be difficult to work on a translation, but asking for help that I'm really sure most people would love to hand(I myself I'm helping a dude constantly to test a porn RPG and even helping him correcting words, the game it's in English but it's not my first language, still i don't care to help).
2) Dl works the same as steam for what i know, so they just take money in fees in base on your sales.
3)I know japanese it's complex AF, that's why I'm saying about a professional, still there's ton of people who translate Japanese stuff.
They'd be happy to translate it in english if lots of english reviews are been left on dlsite or other sites, or been indirect contact to support the developers financially. If you don't and just be like "I'm willing to help", literally nobody will be willing to translate it if you just "asks for it" like a bloody child.
Do you even know the amount of effort and time that you need for a good translation? Even for professionals? I myself have been translating Captured Adventurer (yamada workshop) and the latest work by M-gentlement after-party, which both are brought from dlsite with my own money, and it always took me at least 3 hours for a single scene, and not to mention the testing to see if the text fit in the text box perfectly or any grammatical issues, and since I'm not a native speaker for Japanese, I have to go through some lines again and again just to figure out if this is the right meaning. and do you expect all that time and effort just for asking help?
Also do you think the game really has large audience for the english speakers? yeah maybe, and will the majority of the "large audience" actually go onto dlsite and buys the game directly from the developers form sites such as dlsite? or search online for "free english versions"? think again yourself, either you go translate it yourselves, or stop being childish about the developers not adding another language.