D B Campbell The Roman Army in Detail: The Problem of the First Cohort

[Renatus]

Hyginus does allow 15 feet for weapons and 8 feet for pack animals

You have been misled by De Voto's translation. If you look across the page to the Latin, you will see that Hyginus allocates 5 feet for weapons and 9 feet for pack animals.

I believe is there are two veteran cohorts attached to a legion

Is this a personal belief or is there evidence for it?

thereby making a century have 10 tents, 8 tents for the legionaries, 1 tent for the veterans and 1 tent for the centurion

Hyginus seems to be quite clear that the evocati are placed with the primipilares and the praetorian cohorts and that the extra space in the centurial lines is for the centurion alone.