2) Old Raspbian works just as well as Arch, issue is with Mono and ArmV5 hard float compiled operating system. So basically mono currently is a no-go on Raspbian. A bug report has been filed to Debian, don't know the current state.
I now use mono-2.11.3 (compiling 2.11.4 now:)), most Linux distributions supply 2.10 branch which is the latest stable and support roughly up to .Net 4.0. The 2.11 branch is in development(although been very stable for me) and will be 2.12 when stable. It supports much of 4.5 and almost all of C#5.
Mono isn't exactly on par with .Net, some things are lacking like WPF i.e. and mono has a lot if things not present in .Net
But also Mono now includes some official .Net parts as they been open sources, Entity Framework i.e. is now included.
3) Yes, and that's why I use it
4) Don't understand the question in full here, but as mono is broken on Raspbian it's hard to compare.
5) If you use on of the supported GUI technologies then yes, WPF isn't supported.