benignuman: “bemechilas keovd toroscho”- if you look at the actual teshuvo, you will see that R”Moshe zz’l brings proof that no mechitza is needed by a “chasunoh” from korbon pessach- when families ate together and no separation was required to eat together. So, it seems pretty clear that we are talking about eating together-without separate seating.
As far as your argument about ‘closed family group”, “im kein ein ledovor sof”- it is impossible to gauge each event individually and, in any case, at the korbon pessach, we had different families eating together,not just one family.
Lastly, I looked up the teshuvoh you mentioned in even Ho-Ezer 4,60. R’Moshe zz’l does indeed say that talking to a woman -same as embracing – may come under the issur of “lo sikrevu”, but it is only when there is a real relationship between the two people. I don’t see where ‘kalus rosh’ is an issur medoraisa. I read the teshuvo diffenretly than you do.