Help, advice needed!

[WVduckchick]

If the mom was sitting on them and then you put them in the incubator for 20 days it is very likely they are close to hatch. Mallard eggs take 28 days that would have started with the mother's sitting. If they are true wild the mother would be brown. If you mean brown, but not like a while mallard then the mom is most likely a mallard derived domestic breed.
As far as when to place in lockdown, candle once a day. When you see an internal pip lockdown.

Yes. This. ^^