So I've been fiddling about with the design a bit - unfortunately it's now a bit more boxy looking as the angled sides had to be removed to save weight. I've calculated the total weight to be about 13.46Kg, which includes 300g worth of wires/connectors etc, so fingers crossed it actually ends up within the limit.

Was never really convinced by the wimpy looking stock bolt that would connect the car seat motor to the wedge, so the plan now is to chop that curved end off, and drill a new hole for a 10mm steel rod. Bit concerned that the part which moves along the threaded rod would bend, since I don't know what it's made of and it's quite thin, so I was thinking of reinforcing it with steel U-Channel or bent Hardox from KCut:
IMG_4030.jpg CarSeatMotorNewHole.png CarMotorToWedge.png

Could anyone recommend if this is likely to be too weak or overkill or actually ok?
Thanks,