Interesting. I've ordered the first two RMU books, but haven't received them yet. I'm also coming from a primarily MERP background, though we used a few RM supplements over the years. So I was kicking around campaign ideas when it occurred to me that the answer lay in my old MERP stuff. I wanted a campaign set in a northern region with dark forces looking to revive a long lost evil and the scattered forces of good struggling to resist the rising tide of darkness. And I was thinking of a sort of shamanic Native American/Alaskan feel rather than Norse, but with some fantasy elements. Then I remembered the MERP campaign setting for the Northern Waste. It's perfect for this. You've got Eskimo types (Lossoth), the Umli half-dwarves, sweat-lodge vision-questing Noldor, Snow Elves, remnants of Morgoth's old fortress and allies, and the rising power of the Witch King. So there's our backdrop.
I always thought that the greatest strength of MERP was in adding stuff around the fringes that wasn't canon, but gave you the freedom to game in Middle Earth and bring in the tropes and foes you want to. This is far different from the other Middle Earth RPGs, that want to slavishly reproduce the films or books and set things at the end of the Third Age.
So now I think we'll be doing just that. Using MERP as our base, but adding bits from RMU for depth and customization. Can't wait to get started. So my advice to the OP is to simply fudge it. I think you can easily move stuff from one to the other. And MERP adventures usually have RM stats for key NPCs. Do you have anything with stats for Beorn or some other shapeshifter? Even though it would be an older edition of RM, that could give you what you need.