Some cancelled flights show times in red - does this correspond to the time at which the flight was cancelled?
Some airlines seem to use replacement flight numbers to catch-up on cancelled flights e.g. http://flightaware.com/live/flight/UAL1756 . Note that in this case the flight number appears to be used for any sector and any aircraft. Is there a way to identify these (random) replacement flights?
Is there and easy way to assemble the whole chain of delay/cancellation/replacement?
All UAL flights with 17XX flight numbers are extra sections.
But to your question, there is no easy way to assemble it because the process behind it is extremely complex. To re-assemble that process with secondary information after the fact is even harder.
Each airline will have a different number range they use for extra sections.
Not quite, of 1700-1799 only 1745-1764 are extra sections. Below that are scheduled flights, above that are ExpressJet charters and positioning for charters.