A lot fewer ECS manoeuvres generally need to take place out of the Norwood end of Selhurst Depot in the early hours of weekend mornings, which means overnight works in the Norwood Junction area are rather more convenient on early Saturday and Sunday mornings.
The Selhurst end of the depot can accommodate some but not all stock moves - only if the Slow lines on that side are open for traffic.
As for running a service on the Fast lines between Norwood Jn and New Cross Gate... it is actually technically possible, but it would need to run as a separate shuttle to the core ELL service. As passengers would need to change trains and platforms, it might not be quite as viable as a through service. You also have to bear in mind that the tracks on this route are paired by direction not use, therefore any worksite on one line in one direction may also need to impact on the other line in the same direction, which is immediately alongside/adjacent.