Great questions! Anything you make with Kiosune Studio is 100% yours — commercial use is fully allowed, no royalties, no non-profit restriction. Credit isn't required, but a "Made with Kiosune Studio" mention is always appreciated and helps the tool grow. The engine itself stays free; you just can't resell or redistribute the tool as your own.
NSFW games with consenting adult characters are fine to make and publish on itch (mark them 18+ per itch's rules). The kiosune.com library is curated SFW-only, so they won't be listed there — but that's the only difference.
Also, heads up: KiosuneVN is coming soon — a paid bundle of Kiosune Studio, the Character Creator, and Kiosune Audio, with GameMaker-style tiers, launching on itch and Steam. Same ownership terms will apply to games made with it.