Hiya "he-who-tilts-at-windmills"

I am not going to answer all your questions ..
reinob can do that far more succinctly on the topic.

(speaking of which....THAT was a fabulous layman explanation reinob...really truly...fill this place with explanations like that and we can then teach "maemo linux and how to use your devices like a pro..." classes to kindergartners without having to teach at all.. A truly excellent way to help new people understand ..layman to pro ...very cool ... 10 stars man...really.....I now need to find every post you've ever done and blanket "thank" them...)

I will however address your biggest concern...
which is a valid concern at that..
roasting your primary daily driver because you fiddled with it...
yes it is possible to roast it...
more likely to happen is the need for a reflash and reinstall which can devastate you temporarily and still leave you phone-less until you get it up and running again.

The solution to this dilemma is simple to alleviate your stress and worry ...so you never have to worry about your primary phone.

A- get a second beater n900 and fiddle with that instead.
and / or
B- get a n800 or n810 and fiddle with them (may wish to wait until you see one on a good price) they are cheaper and almost un-roast-able ....
As you see from my sig I have a number of each of the n900 n800 and n810...for just such purposes..
I have 1 of each.. which is my dedicated daily driver..
I have 1 of each ..for experimentation and blowing-up purposes ( )
and
I have of each...tucked away as backup devices in case anything tragic were to happen to the 1st two.
I have had my n800's and n810's coming up now ...for almost a decade ...
Superman got nothin' on 'em ...Almost freakin' Kryptonite-proof

So yeah..experiment with a precursor to the n900 if you are concerned about your daily driver...or get a 2nd n900..
only way you aren't going to ever NOT be worried ...