Hi, am not sister Kang but I may be able to share some info.
Feed on demand is like when baby cries for milk, you feed but since you say u bottle feed baby, would suggest you to pump every 3hrs to maintain your milk supply.
It is always advisable to pump at fixed timing so that a signal can be sent to the body to produce milk.
It will not exactly kill the nutrients but do take note that it should not be boiling water. Just lukewarm water would be ideal.
After warming up, needs to be consumed within an hour.
To me, so long as breastmilk is warmed up, need to consumed within 1 hr, Discard any leftover milk.
As mentioned, for all breastmilk, so long as it is warmed up, MUST be consumed within 1 hr else please THROW away to avoid having bacteria infection.
FOr pumping, usually will pump abt 15 mins for each breast and it is usually enough unless your milk supply is alot and you may require longer time.