The closed cell foam he's using is already a vapor barrier, even more so with the foil. As others have said, stacking barriers is not a good Idea. Depending on what you use to insulate the ceiling, you may need or want one there.
The peicemeal approach should work well with all the gaps foamed or taped or other approaches to limit the pumping, but no envelope is ever perfect. Spray foam gets close, but many still "leak" air because of cold laps or pinholes. It's a shop/barn and not a house so the risk is already lower. If doc installs a mini split, he'll be able to normalize the temperature enough to avoid pumping moisture through the walls.
The studs will definitely bridge and conduct heat, but houses were built with thermal bridges to the inside insulation for decades, still are. It would matter a ton more for a living space than a shop. Personally I'd be trying to stagger studs or do something to improve the insulation performance, but the juice may not be worth the squeeze either. I doubt the goal is to keep the place at 75 when it's 5 below after all.
I'm curious what the plan is with the trusses, because I can't imagine it would be doable unless the whole roof and current trusses were pulled out and a whole new roof assembly installed. Trying to field convert standard trusses to scissor is a tall order and huge risk. Trusses typically need an engineers stamp and field built aren't really a diy endeavor for someone who hasn't trained with someone with experience. Not that it's impossible, it just isn't simple either. The scissor configuration will need to be designed to work with the roof design and the fastening at the splices are critical. Mending plates aren't rated the same as the connectors used by truss makers. The hydraulic tooling used to press in truss connectors also isn't super cheap.
The gaps at the roof to the walls are going to be a challenge. They do sell sealing strips that match the metal roofing contours and you could mount those to a 2x and then nail them in to close everything up decently before coming back and spray foaming the small gaps. Without exterior baffles, soffit venting is asking for birds and mice to take up residence in the attic. Gable venting for a shop should be fine, worst case you'll want an attic fan. I'd be tempted to install a "whole house fan" anyway, since it would probably be handy - especially in the summer. It would also provide a way to clear dead air in the attic on occasion.
@dowboy1970 makes a good point, if there's no vapor barrier under the slab, you're likely to get moisture wicking up through the concrete which will raise the interior humidity every time you warm it, and then you'll get condensation every time it cools off. There are products which can "seal" or cure concrete (not paint, paint won't stop moisture!) and can help lower how much moisture can wick through. It's also cheap. I forget what the products are called but they advertise that they seal and harden concrete further. A good moisture test is to put down some plastic and weigh down the corners for a few days. If the concrete is dark or wet when you lift it after those few days, then you're going to have trouble. No dark spot? You'll probably still get some moisture, but not too much to deal with. I've seen people install radon fans to help alleviate moisture issues through a slab with decent results, fwiw. Dehumidifiers can also work well when the moisture load isn't excessive.
