Haha no worries, me and my mate had issues and worries when wiring mine up so rather than you do the same i thought id give you a heads up. Yeah if the plate you weld to is part of the chassis then mounting onto that will be fine as it will allow the igniter to earth. 1) Little witches hat is air intake temp sensor. Ideally wants to be as close to the air filters as possible as the ecu adjusts itself to suit the air temp. Mine is inside the filter housing. 2) shouldn't be a problem if in doubt you can always run a wire from what you want to earth out to an existing earthing point. 3)the bottom of the bay to the side? Or the engine to the side? If its just from chassis to chassis no that seems a bit pointless, unless it was a string of earths along the chassis to the battery, but id only do that if the battery was in the boot. Engine to chassis yes. Id expect about 2 or 3 earth straps from engine and gearbox to chassis or direct to battery on one of them.