Traveling to Edinburgh w/a connection through Paris (CDG)--Passport Control in Paris?

If you booked this all on one ticket (i.e. a ticket USA-CDG-EDI and return), you do not have to go through passport control in Paris, but you do have to go through a security check, since CDG generally requires connecting passengers arriving from a non-Schengen country to go through security screening to catch their onward flight. You will then clear (British) passport control in Edinburgh.

If you didn't book this on one ticket, i.e. one ticket from USA-CDG and then another from CDG-EDI, you may have to clear passport control in CDG depending on a couple of things. If you have bags that aren't checked through to Edinburgh, then you'll have to pick them up and re-check them in Paris, which will necessitate your clearing Schengen passport control to get to baggage claim. You might also have to clear passport control if you have trouble getting an onward CDG-EDI boarding pass and you aren't able to get this in the airside area.

Not sure how Briexit will affect this if it is in place at that time.

Not sure how Briexit will affect this if it is in place at that time.

No different to how it is now.

The magic word is "Schengen"
The Schengen Area is a "Passport Union", you only go through immigration (passport control) when you enter or leave the Schengen Area, not when travelling between Schengen Area countries.

The Schengen Area is roughly the same as the EU, but there are some differences. The UK is not, and never has been a member of the Schengen.
If flying from (outside the Schengen Area) --> Paris --> Edinburgh, you will not go through passport control in Paris, you will stay airside. You will go though passport control in Edinburgh.
Brexit will not make any difference here.

Map of Schengen Area: https://ec.europa.eu/home-affairs/sites/homeaffairs/files/schengen_map.jpg
If you, for example, flew (outside the Schengen Area) --> Paris --> Rome, then you would go through passport control in Paris, but not in Rome.

a) You will go through passport control when you enter the UK i.e. at Edinburgh.

b) What happens at CDG depends if you need to cross the French border for your second flight. If it’s all one ticket, with luggage checked through to Edinburgh, – no you simply make your way to the gate for your flight to Edinburgh.

c) Passport control at Edinburgh won’t take 3 hours.

Thank you, all! Your insight has been very helpful.