UPDATE:
I took it in yesterday, they fixed a lot of issues with the leather. It does look a lot better, not as good as I'd like it but:
It seems the leather trim is not perfect, and in fact, I can see that the foam is 'too big' for the leather in some places, and that was what was causing some of the imperfections in the finish.
For example, the 'tips' of the bolsters were being 'crushed' by the leather trim, and that was what was causing the creasing of the material that was visible down the side of the bolster, it was the foam compressing and bunching up.
The hinges are still exposed, and he said there is nothing he can do about that. It genuinely seems that the seat-back cover is too small for the foam/seat frame.
The imperfection around the airbag cover is also due to poor stitching. This was pointed out by Blackman's person, and concurred by my father (10+ years as a seating engineer in the automotive industry), and he is definitely not one to let something slide, in typical ethnic immigrant fashion.
I believe they have done the best job possible with what they have been given. And they did not charge me any more for any of the follow up work.
Regarding the plastics:
They seem to have fixed the release lever (in regards to how it seemed dislocated), but otherwise, they did not do anything, but the plastics seem to be reasonably retained, so I am just going to settle for this, I am just glad that overall the seat cover looks a lot better (at first glance), and really, as best as it can, I believe.
I am just glad to put this whole ordeal behind me (as there was more to the story prior to this, including what resulted in the requirement for a new seat cover... but I won't go into that).
I have to say, the owner was reasonable when I came in to speak to him, and handled it quite well, so props to him. I think most of the time it takes a polite, face-to-face conversation with the person to get things sorted.
Overall regarding Blackman's, I would say, not ideal that they broke clips of the plastic trims, and that the leather had grease marks on it, which it still has, and this was a bit of a disappointment. But, otherwise, I expect that they actually specialise in trimming up seat covers from scratch, which is a different process to what I asked them to do, and perhaps this is where their experience lies.
Thank you once again everyone for taking the time to give me advice and support on how to handle this.