You're right in stating that API-650 Appendix P isn't applicable to your nozzles. Without performing FE analysis, your best option is to try to use WRC-107, as you stated. Of course, this assumes you don't exceed any of the dimensional limits that would put you out of the scope of the 107 bulletin. If that happens, then you'll have to make some assumption, or bite the bullet and have someone do an FE analysis for you.

What ever you do, make sure you document it. That way, if someone disagrees with you, they can offer a better solution. At least you told them up front.

You're right here also. But if you read the references for this appendix, you'll find that Appendix P was developed from two technical papers, discussing the instrumentation, testing, and subsequent FE analysis of two full scale tanks. The limitations (in the appendix) exist because of the limited amount of testing.

You also questioned if the API-650 flexibilities are applicable to your situation. I'm going to lean towards "no", and suggest the WRC-297 nozzle flexibilities (again watch dimensional limits).

I hope this helps. Maybe someone else has something additional to offer?