Ok, I understand. I'm sorry but if you followed these steps and your device is in R&D mode, I'm not able to help much as I installed it on memory card partition with no problems (haven't checked too much, but it booted one time few minutes ago). Maybe it really is a matter of internal/external memory... What I would do in your situation is trying to repeat the whole process, and if it still doesn't boot - give up or try to put it on sd card (even borrow a card from someone for 1 day and see if it works this way to be sure it's because of using internal memory).

And three more questions/suggestions:
1. Did you install/reinstall some kernel related packages afer copying /lib/modules and /lib/firmware directories to ubuntu partition? I guess you are aware that you should after every installation or upgrade of kernel, but I want to make sure :P (and I still have no idea why you cannot boot Ubuntu, as it looks like you did everything allright, so I'm just guessing)
2. Did you check the permissions of /lib/modules directory and /lib/firmware if they are the same as in Maemo? (these suggestions sound noob'y, but it's never bad to check... we are just humans and everyone makes mistakes, right? )
3. If all fails, maybe try some older NITDroid kernel? (it may be stupid idea, but someone in the thread about Ubuntu mentioned he booted it with NITDroid kernel around a year ago, so maybe (without installing deb, just unpacking files) copy the /boot/blablabla kernel file (05 instead of 06 maybe?), /system/lib/modules/blablah.NIT.05 to /ubuntupartition/lib/modules and create custom multiboot entry for it? I would try that also in your position)

I'm out of ideas now

Edit: oh, I see you already have vmlinuz-2.6.28.NIT.05 kernel installed... Did you try that? Maybe try 04 or older then?