(a) I didn't like that Barnabas was willing to go along with Lang to use Jeff's face for the head, but I am glad that Barnabas had a change of heart after hearing Vicki's concern for Jeff. As selfish as Barnabas can be at times, you have to admit it was nice to see him be selfless in this instance and decide not to sacrifice his rival for Vicki's affections.
(b) I wonder if Dr. Lang really would have gone through with shooting Barnabas. If he did that, then who would he have used to have brought his creation to life? Barnabas would have something to gain (his survival and being free permanently from Angelique's curse) from doing this, but no one else would.
I agree that Addison Powell was a bit too over-the-top in here.
(a) More should have been made of this decision, dramatically. It was at this moment that we see Barnabas reform. Not completely, but not killing someone he doesn't even like, who is his rival, when his perceived self-interest is threatened, is a radical step for him. If only he had cried "enough" more loudly, instead of muttering...
(b) Lang did not begin his work certain that he'd find a vampire to take part. Without Barnabas, Lang would have gone ahead in whatever way he'd planned on in the first polace, perhaps just animating Adam.
Then of course, he would have had a brief celebration at the success of his work and then gone into aluminiuum siding, since he couldn't publish his work without getting arrested...