AFP. TimeMachine backups via SMB is a feature scheduled for the version 6.7 release of unRAID.
As far as I know, it doesn't matter.
I use one share per Mac. Lets me limit the size of each backup.
No. You should be able to just mount the share on your Mac and point TM at it.
I am far from an expert, but it's my understanding that it is best to set an alternate location for Volume db path. I set up another share for this-