The etymologies in Webster are speculative in this case, I think, as is often unavoidable. Since 'marrow' in this usage is Northern British rather than solely Scots, I'd guess that OE is the more likely root; but that too is of course mere speculation. I don't have a sufficiently extensive OE glossary to locate either 'maro' or 'maru', and 'maraon' ('together, in concert; lit. as one') is in Dwelly but not MacBain. Whether there is any real connection between the OE and Gaelic words I am not in a position to say.

'Marrow' (as in bone marrow or vegetable marrow) appears in recognisable form in enough languages -including Sanskrit- to suggest an Indo-European root (likely 'fat', I gather) but there is really no way of knowing if the two words are anything more than just homophones.