Is DLM smart enough to know it’s a time switch cutting of power to the Router at midnight for 6 hours each night – and not a ‘line’ fault?
It's not smart enough to know if you powered the modem off, if the power was cut or if the line was pulled out the wall.
It doesn't take any notice of the "dying gasp" (a message a modem sends to the DSLAM to say "this was a manual power off").
It treats them all the same.
DLM doesn't count the number of times a line loses sync, or the number of times a line establishes sync.
It counts the number of times a line resyncs, only when there was activity detected on the line in the previous full time bin (a bin is 15 minutes).
So if your line is powered off for 30+ mins then the DLM doesn't count the line syncing back up as a retrain.
You're allowed numerous retrains a day anyway so you shouldn't be worrying about powering off the modem once a day.