Right - you may have seen me mention on a couple of posts that I intend on flashing my friends .bin file onto my ECU.
Don't.
Aside from the checksum checks, is there anything else I need to be wary of?
His file shouldn't need checksum'd if it's cosher. Verify with ME7Check
Will the immobiliser cause me any trouble, or is this not a problem?
It shouldn't be an issue, unless SW versions are different.
If I use another ECU, and swap this into my car, will I need to code out the immo then?
Adapt the immo with the car. Or clone the ECU and then you have two.
Flashing maps unknowingly and cross-flashing is not good.