Langka can help, but its success will depend on how well the paint matches. In your case with the metallic silver it can be very difficult. Get a factory touch up bottle if you can.
I would ask the guy who installed the clear bra to remove it. This way the scratches and marks can only be blamed on him. You have to worry that he could blame the spray shop for doing some damage, too. (Unless he has already seen the damage and admits it.) This keeps your "case" cleaner.
I would definitely try to compound the knife cuts and perhaps wet sand it with a very flat, stout, but flexible backing material. I like using red and white block pencil erasors. Cuts with a knife blade are a little different than regular scratches. Scratches remove the paint. Cuts cause a "split" in the paint like a "V" shape. The outside edges of the cut are now raised slightly. By pushing hard against the cut and then following up with a careful wet sanding you can flatten the raised edges and make the cut less obvious.
Paint touch up is next. The cut will not hold much paint. First be sure you have the cut cleaned out so it's not "black" as you state. Do this with polishing abrasives. Use a foam applicator only...better than terry covered. After laying in some paint you need to let it cure and then remove any "blob" with Langka or use a polish abrasive and a flat backer wrapped it a smooth material like a piece of bedsheet/pillowcase. A T-Shirt material is almost too thick. Thinner the better. This will keep the abrasive working the high spots only.
I like clear bra a lot and have it on all my cars. Trimming it requires the installer to just scribe the film, not cut through it. He obviously was at fault. But fixing the knife marks best as possible and reinstalling the film properly will likely hide the marks. IMO clear film is a better option than a vinyl bra. The film is on your car 24-7 and doesn't change the looks of your car. Don't give up yet!
:up
