Hello Callwatch,
Thank you for using Apple Support Communities and for sharing all these details with us. We understand how important it is being able to answer phone calls on your Apple Watch. You've already done a tremendous job troubleshooting this specific issue. We'd like to check a few settings and recommend an additional step you can try. First, you'll want to make sure that your cellular plan is showing up correctly on your Apple Watch. Follow the steps below:
1.) On iPhone open Watch app.
2.) Tap on Cellular.
Does it list your cellular plan there? If it doesn't, follow these steps on how to add your cellular on your Apple Watch: Set up cellular on Apple Watch - Apple Support
You might also want to install any pending updates your iPhone might have. Including your carrier settings:
1.) Update your iPhone or iPad - Apple Support
2.) Manually update your carrier settings on your iPhone or iPad - Apple Support
If you continue to experience issues, please consider reaching out to Apple Support. They might have to investigate this issue further with you. Use the following link to reach them: Get Support
Thank you for using Apple Support Communities. Best of luck.