If no one can repare those carts why they are all willing to check them, just to capilalize on inspection fee to refuse the work then and to send the cartridge back to the owner? I don't think so. This is a very well known models and each retipper knows how it works. I have never ever contacted Van Den Hul, but the Expert Stylus are also manufacturer and then i described by problem with AT-ML180 they are willing to check it out. Maybe you're smarter than all of them, but if they are smart enough they should refuse any offers to rebuild those carts from the start. Or they are dishonest and willing to capitalize on that small inspection fee?
What exactly Van Den Hul does with all those p100 mk4 carts is still a top secret, but he did the job and somehow got paid and the owners are happy.