[ekys]

I think you're missing the most fundamental point when thinking of time as linear.

Time is linear from the observers point of view.

The diverging universes theory maps every choice as a fork in a road - so if you could stand back and look at all the parallel universes, you'd see time as almost infinitely branched.

So if Rose and the Doctor went back and caused Tourchwood's establishment - that event becomes part of the linear string of all the future branches, from the point of view of whoever is looking back.

so if you think of it like this

.this is just for filling space........ !---------------Rose/Doctor (Parallel 1)
Queen Vic ---------(divergence) !
.this is just for filling space........ !---------------No Rose/Cybermen (Parallel 2)



So if you stand in either Parallel one or two and trace back to Quen Vic - it looks linear.

The thing is - no matter how many other branches happen - when you trace backwards from the observers point of view, time will always be observed as linear. At any given present time - if you look sideways in the time map, you will see your parallel universes along side you. It is only when you go forward that time is not observed as linear, but highly branched.

This begs the question, how the Doctor can expect things to be a certain way when he travels forward in time, seeing as parallel universes can only really be viewed from the present moving backwards and not forwards (as they are still diverging).

One possible explanation, often cited in Sci Fi, is that time, while offering innumerable chances to branch out, will always have the same "basic shape" in terms of history - so if you went back and killed Hitler as a kid, someone else would take his place and a WW2 of some form would still take place.

It is all very brain melting to sit and think about. I found pratchett's nightwatch, to be a very good piece of writing in terms of getting the whole concept across in laymans terms (and it is a good book).