Re: K9's (And Romana's) return from the CVE?
Their ability to travel through the CV was covered in Terraqnce Dicks' 7th Doctor novel Blood Harvest (which relies on there being a conversation between the Doctor and Romana before he departs that can't happen in the screen version of Warriors Gate, but is there in Steve Gallagher's novelisation). I don't think ther's ever been any explanation of how K-9 can work back in N-space though.
"Some days are better than others. They say that where I come from."
"Loudly, I imagine, on the day you left."
(Blake's 7 - Rumours of Death)