There are folks more qualified than me to answer this, but since I'm here and you have no replies... 🙂
First thing that I found helpful when I got started with NetApp storage was reviewing TR-3749 which focuses on storage best practices between NetApp and vSphere. There's a comparison table that shows the features of each type of storage protocol.
As far as question 1, I'm not aware of too many scenarios where you are required to use RDMs although 1 example is Microsoft clustering. Even though VMware seems to recommend VMDK as the default option, NetApp best practice guides for things like MSSQL and Exchange use RDMs, which I believe stems from integrating with SnapManager. As far as your query about SnapDrive, my understanding is that it uses physical mode only, but I haven't checked this issue out specifically in the last couple years.
I haven't attempted to migrate RDM to VMDK, but I will say that VMDK formatted disks won't have the SnapDrive management and operation features if you're reliant on them. But then I'm not sure what services you are running, so my comments are unfortunately a bit generic...