Regarding backward compatibility:
Just got a mail from my hoster today: They are blocking all J1.5 sites at friday, because of massive hacking attacks and unstoppable server consumption So, are you really going to keep J10 compatibility? Maybe its time to drop support for all newer UddeIM releases. Almost all Joomla extensions are 2.5+
(o.k. except for CB ) But that will also change with CB20, right?!

A. ) would like to try it! But currently busy with other stuff.

B. ) Do you really need to change the code? Adding bootstrap classes shouldd not harm previous styles and systems, IMHO.

To support Bootstrap you need to make a specific UddeIm template changing the code shouldn't be necessary

I think the reason why Joomla! 1.0 and Mambo 4.5 is still here in Uddeim, it's because slabbi hasn't not very much time to rewrite entirely Uddeim...

Unfortunately this is correct. UddeIM uses very old code (working and well tested but quite impossible to maintain). It is easier to rewrite uddeIM completely,

It will be time consumming to rewrite it in MVC and use fonctionnality from Joomla!