V6 multivalve AYL motor timing chain sprocket problems

[ESS 0]

My V6 Sharan with the multivalve motor started then stalled. Ross Tech diagnostics gave cam timing improbable fault codes. Compression test confirmed valve timing problem. Pulled motor and gearbox out and dismantled motor. The small sprocket which drives the camshaft timing chain has lost all it's teeth. They have worn away. Motor has only done 133690 km. Can anyone tell me

1) what caused this to happen?

2) what do I need to replace apart from both chains, the sprocket on the intermediate shaft that has no teeth, the guides and tensioners

3) is there an upgrade to the timing chain system that I should install?

4) how do I stop this happenning again?

5) Do I have to do anything else to the motor?

Any help / advise will be appreciated.

Thanks

Alan