Are the curfews the result of any kind of E.U. labor regs? This could cause some real quali problems for a car damaged in FP3.
No - the Working Time law has a specific exemption for people working in places where it would be difficult to relieve them. The particulars vary across the 27 nations, but I believe all of them include international situations where no experts of similar expertise are nearby, as well as for limited-access events where any such expert wouldn't be able to enter due to upper personnel limits. (The UK's exemption list, in case you are wondering, is unusually broad, so leaving the EU won't affect that).
The reason curfew exists is because a few years ago, the powers-that-be finally realised that mechanics were reacting to the creeping calendar creep by leaving the sport. It used to be that about half a team's mechanics would be there for at least a decade - the "lifers". Now, the average is about 3 years, and "lifers" pretty much don't exist in the pitlane any more. All-nighters 16 weekends a year is one thing, but all-nighters 18-20 weekends a year turned out to be some sort of invisible tipping point.
Once the initial 6-hour curfew was imposed, Liberty decided this made the paddock much more civilised, and increased the degree of challenge a bit. So they extended it by an hour and have gradually reduced the number of "jokers" each team gets (you're currently allowed 2 occasions in the year when you can have 4 mechanics stay all night, but by 2023 that's going to have been worked down to no "jokers"). Note that the curfew has an exemption - 10 staff are allowed in an hour early to do tasks that don't require entrance to the pit garages or work on the cars (e.g. cooking people's breakfasts, putting the plant pot outside that got taken in to stop last night's gale blowing it over).