Hey there, @lecoder 
Thank you so much for writing such a thorough post, and welcome to the Netlify Forums!
I know you shared that you read many posts about getting a 404 back. I just want to confirm, have you also seen our forms debugging article?This Support Guide is the first port of call to debug any forms issues, and sometimes folks miss it when they are searching around.
If you have read that and tried the steps, please let us know the form name that you’ve set and that shows in our UI, and we will continue to look into this issue for you!