Ubiquitin – I agree with almost everything you wrote (except for the fact that if Shimon insists on serving c”y, I don’t think Reuven should argue with him, although perhaps he can offer to provide the c”y).
When I was a kid, we had a frequent Shabbos guest who only ate Yashan (back in the days when that was practically unheard-of). So he used to bring his own challah, and it wasn’t a big deal. He never made us feel like we were less Frum than him because we weren’t makpid on Yashan.
What’s interesting is that many years later, I happened to ask him why he’s makpid on Yashan, and he responded, ”because chadash is an issur d’oraisa”. (Note: I was actually in the middle of eating non-Yashan when he said this!).
Which just makes it more impressive that he didn’t have a problem eating at our house and didn’t make a big deal about the fact that he was makpid on Yashan and we’re not.