Thank you, John!
yes, I'm not very familiar with the Mer project and its history, but I think Mer, Easy Debian and Moebian have similar goals - to enable access to up to date open-source software from Debian.
Each project may use a different approach, but we should share as much knowledge and work as possible.
Instead of starting from scratch, we would start with copy of Maemo5 that we can modify without bricking our devices and fix the opt problem by moving root to eMMC/SD.
This would be already a small but useful achievement.

Then we would upgrade the essential components and if possible, send necessary Nokia patches upstream. I guess most critical modifications, which Nokia did not send upstream, affect Glib/Gtk/Hildon? In that case we could keep two separate versions of Gtk/Hildon and link the Maemo apps with the Nokia hildon, while we try to port as many as possible patches to the Debian hildon.

I don't have the time and energy to fix all flaws of Maemo, but we should merge as much as what is possible with reasonable effort, and keep the rest separated.
In the worst case, we'll just be able to install only non-GUI applications and libraries,
which good enough for me, because GUI apps would need to be modified to work on the small screen, anyway.

While trying to upgrade some Maemo components to the latest upstream I noticed that they are often based on so outdated Debian versions that it was impossible find the original Debian sources to find the Nokia specific patches in the diff. How did you extract the Nokia modifications?