ruimen is transparently fire + place, so supplementing men (which is not found in S?) would make sense, but isn't that rather sad as is Calenhad?

There is the sundóma suppression rule, in particular when the base vowel is prefixed (VT47:13). It produces CE forms like ap'lata > ablad (PAL-), aklar > aglar/alkar (KAL-, for some reason no apostrophe) or as'tāra > astar/astor (PE17:183).
Perhaps it applies to compounds as well, so we would have *ruys'ta > ruist.

Calenhad is a much later compound - it has to be, because of lenition instead of nasal mutation.

This would imply that "ruist" would be a very old compound. Alright, I'm adding it to the Neo-Sindarin list. Not exactly a coined word, but it is slightly shaky as in it's an assigned meaning.

How is it an assigned meaning? The translation 'fireplace, hearth' is right there.. An unclear etymology doesn't make a word Neo-, otherwise much (most?) of English would be Neo-English. :-)

It's not given in any Sindarin dictionaries anywhere else, so Neo-Sindarin users without access to PE17 would otherwise not know about the word. I only found it because I went looking to reconstruct it, because it seemed that Sindarin didn't have a word for it. I don't have any dictionary websites l that I maintain other than contributing to this one, so I put it out there in a place I could reach.

+Fiona Jallings "It's not given in any Sindarin dictionaries anywhere else, so Neo-Sindarin users without access to PE17 would otherwise not know about the word."

Except for German and Hungarian users, since ruist is given as an attested word both in "Sindarin Lexikon" and in my dictionary. :)

Ah, should have specified: it's missing from English-Sindarin dictionaries. Unfortunately, I speak neither German nor Hungarian...

It's in David Giraudeau's PE17 Sindarin wordlist, though he doesn't gloss it.

Yup. That's the problem.