You could use wood hardener, has similar effect to hot water treatment without the shrinkage. Does leave a glossy coating which can crack under impact, but the surface can be sanded without weakening the leather.
Easiest solution is ditch the volkswagen and get a landrover. An ex military ambulance should give you similar carrying capacity and it's unlikely to get stuck.
Only ever used lemon juice for cleaning leather, it's cheap and easy to obtain. Does a good job with no harmfull effects on the leather. Can sting a bit if you get it in a cut or splash it in your eye, but otherwise no harmfull effects.
There's a good rescource for patterns here . Most of the patterns are intended for constrution in steel, but canbe done in leather (and a lot quieter than bashing steel)
Another thing to consider is the time taken to cut the straps. Not so important if you are just leatherworking as a hobby , but if you are selling stuff the extra cost of pre cut straps may be less than your hourly rate