When you power off a VM all open files will be closed and all filesystem information will be written to the vmdk.
When you only suspend a VM some of this information will be stored in the vmem-file instead.
This means that a backup of a suspended vmdk always is a little bit corrupt - most of the times that will be harmless but it can go wrong as well.
So if you have the choice - backup your vmdks when the VM is powered off.

2. You can easily answer that question yourself - just check the timestamps after you power off a split vmdk. Only one or a few slices will have the latest timestamp.

3. as long as your vmdk is healthy you can convert between split and monolithic at any time you want