Hi ROODvogel
I apologize, but point 2 is not possible with ATUM. So I recommend to use custom development of achieve it, but as we don't offer this service you will need to hire a developer.
Regarding the issue related to the stock quantity going from 1 to 1,1 tot 1,2000000000000002 to 1,3000000000000003 I'm still not able to reproduce it. So please, would you be so kind to change your theme by a standard one (as storefront) and check if problem persists?
Best Regards,
Andrea.