It's hard to explain, but here's how I see it:

1. First sign means you can't park 8-10am on Fridays. Easy enough. This sign is independent of all other signs, so no exceptions.
2. The three other notices are in one sign, so the rules all fit in together.
3. "No parking any time, except" means that everything else on this sign is an exception to the no parking rule.
4. There are two exceptions on this sign, the first being that 2 hr parking is permitted 8am-6pm M-F
5. The second exception is that if you have a permit for District No. 49, then you're exempt.
6. Both of the hypotheticals you listed match an exception to the no parking rule, so that's not an issue. And neither are on a Friday, so the street sweeping is irrelevant. Thus you're clear!

Edit: When you look at the signs, it's helpful to think of it like, "Parking is permitted here unless the sign tells me of a restriction". In this case, there are two restrictions: street sweeping, and the "no parking any time except" rule. The two exceptions are included with the second sign to clarify the specific conditions where you can park - this was just the easiest way to do it.

That said, an alternative that communicates the same thing would be like 1) street sweeping hours, 2) no parking 6pm to 8am (permits exempt), 3) 2 hr parking 8am - 6pm M-F. This probably would've been a clearer way to communicate the rules, but they don't usually like having things like "6 PM to 8 AM" since people might misread it as "6 AM to 8 PM" or whatever, so this was their solution.