Too long is when it hits something, usually the caliper bracket or knuckle. What you should make sure you're getting somewhere between 7-9 full turns of threads into the hub for proper thread engagement. Anything more than that is unnecessary.
Stock bolts are 27mm under the seat, but you toss that spec out when you're provided a spec by the wheel company. OZ Wheels are odd, and you're correct about who has that odd bolt. Otis sources from a European manufacturer that specializes in bolts, so they offer "conversion" bolts that work in this application. You'll need to stick with an R12 ball seat bolt, and the 36mm length is minimum without spacers. Every MM of spacer adds that much to the bolt length and so on. You won't be able to use your current bolts for the VW wheels.