As I wrote here, for kp49 you need to download the kernel-power-bootimg .deb file (from the same place you downloaded all the other kp49 .deb files) as the version in the repository is kp48.

I've no idea what you were doing with backupmenu. The stock version doesn't support multiboot at all (though there should be a version posted on here somewhere that does). If you were running the modified version then I can't see why installing NitDroid would break it.