I'm not going to read every line you have written.
If it's your client that owns the app, then they have to be the account holder, to my knowledge. That's the rule change they made around 2018 if I remember correctly. You can have them create an account and give you the Developer role or equivalent so that you can develop and publish THEIR app. If you work for clients, you are supposed to know that.
Topic:
App Store Distribution & Marketing
SubTopic:
App Review
Tags: