This is how I look at it.
You are offering to operate a bus for people. but instead of selling seats, you are putting busses inside your bus.
Whats the use case?! If you're trying to offer a customer resources that they can distribute between...
no.
There is almost NEVER a use case for nested hypervisors except for development/lab use. Even if we assume there are no cpu/ram performance degradation that occurs with modern VT extensions (hint: there are) the consequences of cascading...
You dont. I dont understand the use case enough to comment on the wisdom of the solution; please explain what you mean by VDS, and why you want proxmox inside them.