What could the local congregation do if they learned that either one of the above (faded, ex, whatever) remarries?
"Faded" is the key here. If two JWs separate for anything other than adultery, they are not free to remarry. Ever. Unless one of them commits adultery. Then the one who committed adultery is subject to judicial action (may or may not be DFed depending on how good his story is) and the "innocent" mate is free to remarry. The only way around this is if one or both of the JWs become inactive. If a person no longer is known as a JW, let's say he hasn't gone to a meeting in 5 years, doesn't attend Memorials, etc. then the elders CAN look the other way when he starts doing something otherwise worthy of a Judicial Committee.
Real world factors they would consider, IMO:
1. How "high up" in the WT was the person? The higher up, the longer it takes before they'll look the other way.
2. How much "notoriety" surrounds the person? Did they just fade into the woodwork or was it more "scandalous" in the eyes of local JWs?
3. Does anybody on the BOE want revenge on this person.
Whether or not someone is "still known as a JW" is a highly subjective judgement call in many cases.
I would say 3 years minimum and 5 years is more typical to pass under the radar. But that's no guarantee.
Bottom line: If they REALLY want to DF someone, they will.