If it's "not known for sure", you can't say which one is "correct" for sure.
You would have to research ancient Welsh culture to find out if the combination "marrow famous" was invented to have meaning as a combination. In English, to feel something or know something "in one's marrow" means to feel it or know it in the deepest possible way. But that might not be a metaphor used in Welsh.
Alternatively, it's possible that there was no "meaning" intended by putting the two syllables together. In regard to Old English names, I have read several experts who say that although one can trace both syllables in "two word combination" names back to original meanings, by the time many of the names were created, the words themselves were already somewhat archaic in the language, and the parents were not necessarily thinking of creating a conscious meaning by blending them into a new name. They were just creating a name out of the "conventional name syllables" of their culture.
A famous example of the above is St. Wulfstan, whose father was Aethelstan
and whose mother was Wulfgifu. It's unlikely his parents meant people to interpret his name as meaning "stone of the wolf", but just were blending their own names in those of their son. So what it meant to them was "child of Aethelstan
and Wulfgifu". :)http://www.stwulfstan.org.uk/stwulfstan.html
Again, however, we'd have to find specific information about the Welsh on this issue to know if that's really a reasonable hypothesis in regard to Marvin