The specifics of this situation are vastly out of my depths, so I'll only answer what I can/feel comfortable answering. That said, it sounds like:
1) Ne is very depressed and would benefit from professional psychological help and potentially medication to deal with their* depression.
2) Ne is confused about the role ne wants to play, gender-wise. Ne may benefit from good, sensitive, progressive professional counsel here too. I do not have these feelings, so I'm not qualified to give any advice about them due to lack of personal experience.
3) Ne is some kind of infantilist, and if ne gets horny thinking about it, ne has sexual fetishes surrounding infantilism or diapers, which is not in and of itself detrimental to a relationship, but see #5.
4) Sometimes AB/DL and bedwetting/IC are related. Sometimes AB's like to regress to a different gender (sissy/littlegirl/littleboy). I don't know enough about this to answer that question any more than that, since I don't experience all of these feelings and I am not IC.
5) I think the best relationships have balance. Partners share equal status, make decisions together, and are equally independent and co-dependent. Some time is spent together and some apart. Sex is present in the right amount and is balanced between vanilla and fetish material (and if partners don't have the same fetishes, this usually means more vanilla play than fetish play or is split between one partner's fetishes and the other's). Even a dom/sub relationship is by equal choice of the partners entering it (I think).
I think something you and ne need to figure out is what kind of relationship you are in. Obviously you are both technically adults, but are you in a relationship between two adults who are dating romantically or between a caretaker-figure and a dependent person? You and ne seem to disagree on that point, but I do not know enough to say for sure.
As for what being an AB/DL caretaker means, I think it is up to each couple. I have no experience here. I think #5 counts for something here, though. Your desires and hopes for the relationship are important, and ne needs to know that if the relationship is to work, you need a romantic, dating, adult partner. Demanding you be a single mother is unfair to you, and if it's not the life you want, don't do it!
I think you need to discuss with them* what would make you feel more satisfied and comfortable in your relationship (Maybe you have already).
Also, nothing you said was offensive, don't worry. I hope *I* handled this as sensitively as possible. A sex-advice columnist named Dan Savage covered a case like yours on his column, Savage Love, and put it much less delicately, but the gist of his statement was that the AB Significant Other needs to respect and satisfy their partner's wishes just like the vanilla partner needs to respect the AB's, and it can't work otherwise.
Good Luck. My thoughts are with you, Alexandria. Welcome to ADISC.
*I am unfamiliar with the ne/nir lexicon, so I'm using singular "they."