kernel-power48 can be updated in numerous ways apart from using fapman, you can use hildon application manager (known as HAM), you can use apt-get or aptitude.

Assuming you have restored to the clean backup you made that afternoon and that you have not tried installing kernel-power49 (which is unofficial at this stage). It is most likely some other program you have installed which made your device bricked.

About flashing, as long as you don't flash with the eMMC image (it is clearly noted in the tablets-dev website, even on the filename itself) you won't lose your 1.5GB backup file

When I was referring to combined aka fiasco, I meant that in the filename you see for flashing the firmware (no not the flasher tool, the other one) is noted as combined:

RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM.bin 200174197 PR 1.3 version 20.2010.36-2 Latest Maemo 5 Global release for Nokia N900

That `combined' I call it as fiasco. They mean exactly the same thing otherwise.