[Sony Xperia P, U, Sola, Go] taboo5200:
Yes, the keys are unique to your device and thus, are not provided in any firmware. The keys are stored in a special sector of the phone's memory that is known as the Trim Area. I believe the absolute lowest level firmware required for the phone's basic functionality (

including flashing

!) resides there as well. Therefore, meddling with that can have severe consequences, such as erratic device behavior and even hard bricking with 0% chance of recovery without intervention from Sony's tech support. Yep, it's that dangerous.

You can check for the presence of the DRM keys by going to the phone's Service menu. Open the dialer and type in *#*#SERVICE#*#* (you should immediately see the menu upon hitting the last *. You do

not

need to hit the Call button, assuming you're fast enough to anyway) -> Service tests -> Security. If you see output similar to the following:

MARLIN [[COLOR="SeaGreen"]Key OK[/COLOR]][Active]
WMLA[[COLOR="SeaGreen"]Key OK[/COLOR]][Not active]

then your DRM keys are intact. If they show up as Unknown then they're probably gone.

Yes, you can. Please see percy_g2's post here. Take note that there are many additional steps to take between steps 9 and 10 that weren't covered in his post, see my post here for those steps. It can be confusing and disorienting at first, so feel free to post your queries and concerns here to get them clarified before you proceed (don't make any changes until you're certain you actually know what you're doing).

As for root revocation (e.g. before sending a phone back for servicing), you will need to keep a backup of your pristine stock firmware, downloaded by PC Companion or Sony Update Service and converted to a FTF. See this thread for details on how to create the FTF, and this thread for how to flash it into the phone (don't worry about it being targeted at the Xperia Z, the steps are very similar). Unless you used some exceedingly obscure / maliciously compromised su binary that placed itself in some strange places in the phone's memory (i.e. outside of /system), flashing in the stock firmware is typically enough to revoke root access since the entire /system partition is cleared and rewritten in the process.

I strongly recommend you to keep a backup of your *own*, carrier customized firmware instead of resorting to an unknown source for three reasons:

1) you have no way of telling if the firmware you downloaded elsewhere is actually safe (it typically is, but you might just get (un)lucky and hit the worst lottery ever);

2) your SI number changes depending on the locale of the firmware you flash. If it differs from your actual phone's SI number, you might get some unnecessary trouble if tech support decides to check for whatever reason (and since you don't have root access, you can't fix it up either!);

3) you can use this firmware as a base to create your own trusted, pre-rooted firmware (see percy_g2's post I linked earlier in this section).

With regard to stuff on GB working on JB, that's pretty much a game of chance - some stuff will still work (like the Cosmic Flow live wallpaper - back that up if you still want it since it's not provided in JB), and some stuff won't. I haven't really tried running much GB stuff in JB, so I can't say much here.

Yes, you can gain root access in GB with the pre-rooting method - see the second section above in this same post. You will, however, need to download the GB firmware in FTF form from an outside source (since Sony no longer provides it on their servers) and use that instead of the JB firmware. This means you will probably notice some small differences between your current firmware and the one you flash in.