Mer is not Fremantle. Mer is/will be based upon many components of Fremantle, but also on Ubuntu, rather than normal Maemo. I believe the initial inspiration for Mer came from the question of "how could we make a better Maemo, if we were to do it again?", and, as such, it is bound to be somewhat different than where Nokia is going with Fremantle. Secondly, being a community-run distribution, based on Ubuntu, born from the minds of hackers in the Maemo community, Mer can be many things; yes, there will be defaults, but it won't be difficult to say, run LXDE instead of Hildon-desktop, etc.
So... in some ways, the answer is yes - as far as Nokia is concerned, it's a "Fremantle Community Edition", but the actual experience may be different. At any rate, Mer will make sure your N810 doesn't get left behind, and in the mean time, Diablo should meet most/all of your needs.