... With regards to the CSSU Fremantle/Devuan idea, how feasible is it to update the Maemo5 componenets? In it's current state (GTK2 version etc), personally, I wouldn't endorse it as a Devuan project purely because of the outdated components in terms of security and not meeting minimum version for a lot of apps. If you can update some of the core components (eg GTK2 version) whilst keeping compatibility with Maemo I think it would be in for a better shot.
It's been a while since I went through the Cordia GTK2 parts, but other than being miles behind now in terms of tracking CSSU, there didn't appear to be much different. I'd stick with CSSU and just merge any desirable patches, much easier than the other way around. IIRC there are some for GLX support that might be worth looking at. I have the fun at some stage of going over all the CSSU updates since the last Cordia update to see what ones are needed. The GTK3 version strips out a lot, like home widgets and file chooser for example. The version I'm working on has a different .so name for libhildon so it might be possible to install side by side if I rework the installed include directory. That way you might be able to run legacy Maemo5 apps using GTK2+maemo patches whilst using the GTK3 version of hildon-desktop. I may have to give it a go at some stage. If I can get that working it might be a more viable direction, once I clean up that darn Clutter code. Some of the stacking code is just using _NET_WM_.. atoms and the window menu icon is another atom so that should be OK. Just not sure how/if GTK2 window menu would work with the GTK3 window manager.
Whilst I remember, building hildon-desktop without MCE fails, as it is still trying to access the headers. Not sure if it is just Cordia GTK3 derived version or CSSU as well. Thats what got me into the whole Maemo/Nemo/Mer userland mess in the first place.