BINGO! The issue here of course is that now instead of _two_ components (Front End A and Embed B) you now have four (the back ends must communicate and if A didn't need a back end ... well, now it does).
Now, if you meant "Use OAuth2 in the browser", that's just the original case (you can't authorize if you can't authenticate and it's the ambient authentication that's being stripped when you eliminate third party cookies).
