Allowing them not to create a password and requiring them to let you know what email address they use with social login just isn’t the same. They can still skip having a password if they use a social login. I often refuse to sign in with my gmail account (though less frequently now) and my wife almost always refuses such. Also, the login-with-email is really handy and means that you never need a password. But I digress.
If the API requires a password, just generate a random one. Having a password that you don’t know and don’t need is pretty much the same as not having a password.
At some point I made GitHub - pfaffman/discourse-user-creator: Create an activated user, optionally assigning to group · GitHub; I don’t promise that it works, but it should get you pretty close. You want to create non-activated users, so you’ll need to modify it, but it should be pretty clear what to remove. (If you just want the problem solved and have a budget, please feel free to contact me.)