Calling all armchair generals! Boudica's Last Stand.

[Renatus]

Catching up after a brief illness.

I hope you are fully recovered.

In military terms "position" does not mean "battlefield" but the location of a unit or force on the battlefield.  So depending upon who is "approaching" the defile is either behind SP's position as he is approaching it.  If Tacitus is referring to Boudicca then the defile is in front of the Roman army.

I am not sure that I fully understand this.  Perhaps 'approached' in the translations is getting in the way again.  The Latin seems pretty clear that Suetonius was in the defile, which would seem to lead to something like your second alternative in that Boudica's forces would need to enter the defile to engage with the Romans.  

From the translation The narrow defile gave them the shelter of a rampart.  This is an interesting phrase as a rampart is a defensive wall with a walkway and parapet along the top.

I don't know where this translation comes from.  It is not Church & Brodribb or Loeb or Michael Grant.  Anyway, I would not get hung up on the word 'rampart'.  Munimentum can mean 'rampart' in the proper context but it can also mean 'protection, shelter, bulwark', so the configuration of a rampart need not come into it.  As I see it, the defile was probably defined by high ground in either side, possibly forested to add to the the difficulty of trying to outflank the Roman forces. 

The "wives" of Tacitus may be better translated as "camp followers" again in tribal clumps causing bottlenecks and chaos behind the army.

I don't see any reason to go behind Tacitus.  Coniuges means 'wives', pure and simple.  That is not to say that there might not have been camp followers as well but Tacitus does not mention them.  Nathan's point made some time ago that part of the confusion at the end was caused by British warriors trying to get to their families seems eminently sensible.