The age is not a problem but as far as I know you can't prove that on steam.
Just one question to make sure I haven't misssunderstood you the whole time, is the plan to ever release a full version here? To be honest I'm more happy buying it on steam but just in chase. I already understood what you said about this very same version here.
The NSFW DLC will be free after you purchase the game on Steam - it contains NSFW animations / dialogues / audio etc...
About your second question I'm not sure if there will be a full version on Itch, because I've never thought of it, but by all means on Patreon / Substar there will be a full version and when it's released you can do 1 time payment on there to get it (they introduced "Stores" where you can sell digital goods, I haven't tried it but that could be a good way to do it, or just the monthly sub that can be stopped at anytime).
