[ IJC and Marvin Sketch version should be the same on the machine] - we recommend to use the same version if you install more than one ChemAxon product,
You can upgrade the packages since the dotnet parts are running side-by-side and supports even deletion, however the activeX and COM parts install the latest version only.
So if you put on JChem (IJC, Marvin, ...) 6.0.2 and afterwards install 6.0.3, the activex overwrites the previous version 6.0.2 with 6.0.3 which is compatible by the way, so this is right.
If you uninstall the latest version 6.0.3 of activex and want to continue with 6.0.2 then there is no turning back....
Dotnet is managed by the framework so both 6.0.2 and 6.0.3 package can be running in parallel. I would say that this is not healthy either since you use lot of resources maintaining multiple versions at the same time on the same machine.
I hope my explanation is clear in terms of the dotnet and com packages.
Both ISIS and MDL draw need to be working. Edith you can email me directly.
I try to investigate the issue with OLE ...