let's start at the beginning, why are you doing all of this? are you saturating a single link? If not, then don't bother with it. Things will just become more complex, and chances of it making a difference in smaller environments are close to zero!
1) that is something you can do, but it also means you need to route between the subnets, and most people would isolate their vSAN VLANs
2) Yes, that is my recommendation, keep it simple, unless you have very specific reasons not to
3) You always need a vmkernel interface per host, and those will need a unique IP, but it is easiest to have them in the same subnet
4) multiple vmkernel interfaces are sometimes used by customers who want to create a setup which resembles their fibre channel configurations, it is barely used to be honest, and it doesn't balance load across those interfaces too well, so I would not recommend it.
