Second Contact (Time Signature)

Prose  To what extent has any Doctor Who story "happened"?  If time can be rewritten and the whole timeline is changes anyway every time a TARDIS lands and someone steps out of the doors, any continuity errors or Time Lord memory lapses across the franchise can be excused because fixed points in time only exist when the Doctor (or whoever) knows or remembers what was supposed to "happen" and his big alien brain can't really keep everything on track.  Thanks to Genesis of the Daleks, the first Doctor's encounters with Davros's creations become malleable to such a degree that some versions of AHistory have to account for there being two different continuities before and after.

Although it appears earlier in the anthology, the events of Second Contact are rewritten by something which happens in a later story, so does that mean the Doctor was never in place to mediate this encounter between the native peoples of a continent and a doomed Viking colony and was there even greater loss of life as a consequence?  The TARDIS Datacore suggests this story happens in an "alternative timeline" but Doctor Who has rarely gone in for that sort of thing.  But its also undeniable that the Doctor shouldn't be in this place because his future self changes his past.  Honestly, Blinovitch would have a fit.

Placement:  Given the mess this has made on the carpet, I'm banishing it to the "almost" section.