First thing is, I suggest you update to ubiboot 0.3.5 as it contains lots of bugfixes.

As for running backupmenu, of course the "correct" way to do that is to embed the backup menu under ubiboot, so that it works in the ubiboot maintanance mode domain. That is still WIP, and I might look at it one of these days when I've got some spare time to do it...

What you can do, however is to do what I guess you were after now: Launching bacupmenu in Harmattan preinit context. This would mean chainloading like ubiboot->backupmenu->Harmattan, so that first you boot with ubiboot, and when you select Harmattan you would get into the backupmenu selection before it boots up.
That should indeed be easy, even as I have not checked this myself. Just make sure that the "/sbin/preinit" installed by backupmenu is there, and then set the ubiboot.conf key like you have in a.) - 1.), G_NITDROID_INITSCRIPT="\/sbin\/preinit"

That should work, yes.

Now, as for why your device doesn't boot, when you install ubiboot 0.3.5 it is much easier to debug. You can see what's the problem by looking at the file MyDocs/boot/ubiboot.log.