Scott,
To the best of my (limited) knowledge, there is no difference between the Single & Block registrations other than the location it’s stored in.
In Block Registration you’re limited to 198 locations, in single registrations the sky is the limit (Depending on how big your hard drive is).
To me, the benefit of having the single registration is the possibility of creating folders on your hard drive containing different music styles/songs. For instance, you can create a folder and call it Jazz, and then you can store 999 registrations in that folder. Using the same way you mentioned, suppose there is a song called “DREAM” you can call the first registration Dream “A” and store it in the Jazz folder, and then Dream “B” then Dream “SOLO” then Dream “C” an so on. Then you can switch between them using the footswitch.
And, to answer your other question, you usually select between the “Single” and “Block” mode from the Utilities menu. That way, when the “Single” mode is active, the footswitch toggles between single registrations, and when the “Block” mode is active, the footswitch toggles between Block registrations.
I also think that single registrations are accessed directly from the disk and there is no need to load them in the block registration memory, so you don’t really have to erase a block registration to load a single registration.

I hope this helps…

Maged