Hello99,
Sorry, I’ll respond to your post out of order (I missed it).
I think that the most straightforward understanding of case the R’ma in 253 is dealing with is that there is no other hot food (not the way you misunderstood me earlier that there’s no other food at all).
Also, in 325, he is only mattir items for tzorech Shabbos to be carried through a carmelis, not a r’shus harabim. Why not?
Also, in D”M, he is clearly talking about a tzorech Shabbos (again, see M”B that the definition is something which would be somewhat difficult to do without) and is only mattir if he did it for himself (or it was still warm).
The weight of evidence is clearly that even the R’ma would not be mattir.
I don’t think one would be required to protest if someone had the R’ma to rely on. (Although in this case, it actually wouldn’t make a difference, because it was also bishul aku”m, and, ironically that would probably make it amira l”akum as well even if the R’ma would otherwise be mattir, since there’s no tzorech to have him cook food that is assur to eat! 🙂