My problematic is not so much a matter of grammar, it's a matter of meaning. A green house is not literally a green house and a fire fighter does not literally fight fire. But favorittskjell literally means exactly that, no room for "dynamics" between the two words. I guess it has to do with phonotactics and rhythm. A compound word is easier to pronounce that the two separate words, I guess. At least for me.
But a greenhouse is a special type of house used to grow greens, a firefighter very literally fights fires, and a favorittskjell is a special type of shell - the one somebody likes most.
A "favoritt skjell" (2 nouns), on the other hand, is a favourite whose name happens to be "skjell". Alternatively, I'd interpret "jeg spiser min favoritt skjell" as missing a comma, i.e. "I'm eating my favourite (dish), shell".
This is tricky in Norwegian because the spelling is determined by different factors, of which the most important is the pronunciation. This of course makes it even more challenging when the pronunciation (like when spoken by a machine) is somewhat dubious. However, as a foreigner one would be right most of the times if one choose to write as one word when in doubt, thus this is the most common in Norwegian. Don't forget to consider if the meaning would be different though. ex.: "overalt" (everywhere) and "over alt" (above all).