Rears should wear evenly... a lot of peoples fronts wear on the outside shoulders though...
Fuel clip should have been done by now, would have thought it would have broken down if it hadnt. easy to check, its under the rear seat. Guide somewhere on here
Frnt seat - Probably dont have to remove the seat, the connections are just floating about underneth them...
Yeah, disconnect the battery to allow the airbags to discharge or whatever - just make sure the satnav LED has gone out before disconnecting the battery.
Engine light cant be good? worth getting checked out with a T4 diagnostic or LPG specialist? maybe one of the other LPGers floating around here can comment.