Do the program as written - I suggest getting the Beyond book if you don't have it.
Jim stated in the Beyond book that "the one thing that you will change is that you will NOT go for any extra reps on the last set. You will only do the required reps and move on to the 5 sets of 10 reps." This is for 3 reasons: you're fresher to perform the 5 x 10 sets having not gone to failure on the main lift, you will maintain strength levels by pushing heavier weights on the main lift, and many people actually get stronger when they dial back the last set, i.e., pushing to failure does not always equate to better gains.
NO - "Exercise substitution will NOT be tolerated." You can add in additional assistance work, such as face pulls or curls, but the 5 x 10 sets/reps of the main lifts are a core component of the BBB 3-month challenge.