The circular time idea's a nifty concept too - might go some way to explaining so many familiar faces across the ages
It is and it does.
But your theory regarding Barnabas' actions is probably the closest to the actual scientific theory of parallel universes/time bands.
As the theory goes, every time we're presented with a choice, the possibilty is that we actually take each option, thus creating an alternate/concurrent band of time where each choice is lived out. Because there are an infinite amount of choices in the universe, there are infinite parallel universes/time bands. However, the bands in which the choices made correspond most closely supposedly exist closest to each other, whereas the ones furthest away would be as different from our own "reality" as they could possibly be. If we were to cross into a band "next" to our own, many more things would appear similar than they would appear to be different. This is why so many of the things in 1970PT are so close to how they are in 1970RT.
Just think of it - in theory there could be a whole slew of alternate universes/time bands where DS never existed - as well as a whole slew where it was never cancelled in 1971 and is a month into its 38th year on ABC. And in some of those alternate universes/time bands there could exist alternate versions of ourselves who've never even heard of DS or couldn't have cared less even if they did, while in others our alternate selves might come to a forum similar to this to discuss the current DS storylines rather than those from over 30 years ago. The possibilities are mind boggling - in theory, anyway.