Sorry for the late response - been out and only been able to check replies on my phone!
I'll try to get my codes read and reset tomorrow (have a ODBII-reader that should do the trick, if not I might have to get another one).
I did try to jump start it before I reconditioned the battery - I've read somewhere (Owner's Manual?) about that being hazardous to the ECU, and am somewhat afraid that I might have damaged it. Hopefully that isn't the case!
Current status is:
With the throttle body (I think that's the name of the part) disconnected and key on MAR - no sound, no Check engine, no Immob. failure. Turned the key real quick to see if the starter engine would engage, and it did.
With relay #10 (I think it was #10, have left the garage and am at home now, it's the topmost right relay (12V/30A/red)) pulled out and key on MAR - no sound, Check engine light on and Immob. failure warning. Haven't tried turning the key like this yet.
With throttle body and relay hooked up properly - clicking sound (which comes from the throttle trying to open just to close at once again), flickering Check Engine light and Immob. failure warning. When I turn the key, the starter engine engages, but the engine doesn't start (which is to be expected due to the Immob. failure).
I have the CODE-code needed to do an emergency start, but fail to get in to that mode (the Owner's Manual states that I should put the key on MAR, and press the accelerator for 8 seconds to get the check engine to start flashing). When I first got the Immob. failure warning, I figured that the car had forgotten about my keys while being unpowered for an extended period of time (during winter).
I guess I have a few options - testing a different battery will probably be the first, after that getting a new relay (which seems unlikely, but cheap enough to test). If that fails it's either the throttle body or the ECU that's fried.
Any other suggestions are highly appreciated!