Yes, an immigration lawyer.
And yes, very hard to discern what is what online sometimes. Anyway, I'm really pretty sure about the extended family/adult kids thing. Minor kids only. It's very specific on the K-1 visa page, as well.
Good luck. I hope you find someone good. You should be ok. I've not known anyone to be denied via marriage, even if it does take time. Mostly they look for fraud in these cases... and when it's for real, it's pretty obvious. So it should all work out for you. Hang in there. I believe a lot of it is just dogged determination, and you seem to have that.
And if the whole world goes south, I will retreat to my island and you can move to Jamaica, and we can take the mail boat to visit each other, lol.
waves