If I was CSM, the only way I’d allow someone from J or Y to eat in the F cabin would be if they were going to row 5 and only visited via the rear of that cabin. Its all about protecting the interests of the F pax,
My last F flight from LAX landed in Melbourne Christmas Eve morning. The other passengers were a Qantas captain, his wife and 3 kids aged around 10, 12 and 14, another family of 4 with kids 6 - 8, a gentleman with his 2 year old daughter and nanny and a solo gentleman in his 50's. What are these interests that need protecting? Everyone in that cabin is in there for a reason. If the captain had his kids fly economy and they joined them for a meal wouldn't have caused any issue with me. The rear stairs are roped off and it's all economy both up and downstairs yet J & F can mingle at the front. I really don't see an issue.
Regards,
Michael