Q8: Is it supported to have a different kernel and application build?
YES, it is supported to have different builds for the application and the kernel BUT... they must be for the same version and service pack level.
Our Product Group ensure that dependencies between the application and kernel are not introduced when creating AX hotfixes. Dependencies between the application and kernel builds can only be introduced in a new product version and a new service pack (NOT in a rollup/cumulative update).
We also recommend having a kernel build that is the same or higher build than the application build. Applying a kernel update is normally a simpler process than updating the application code, so in the case of applying a cumulative update it makes sense to apply the kernel part of the update as well as the application part. This also ensures that you get the full benefit from any hybrid fixes, which as mentioned above require updates in both the kernel and application to function fully.