KVM - Which Web Based management to use?

Have you tried Kimchi? is the most user friendly open-source web front end on KVM and it's sponsored by IBM.

Disclaimer : I am not affiliated in anyway with Kimchi. I just like its user friendliness.

Q1&2

For web-based KVM manager, you can try Proxmox. Instruction for Debian Squeeze is here.

It has command-line tools, but not text-based -- menu-driven.

Q3

You should setup a small environment and try it out. Then set out a goal and implement it.

Q4

There is no accurate answer/tutorial for this. How tunable your KVM server/client is really depends on how many resources (cpu/ram/storage) are available.

Q5

Again, this depends on your environment and it is multi-layers/devices, e.g. routers, firewall, ip-tables, password policy, access policy, etc. I am not aware of a single GUI, web-based or not, that can handle all of that including the KVM. (OK, there is cloudmin, but I have no experience with that at all)