LB, that was my question too. I only read the Rema, and it sounded like you only add 1 period, which it sounds like is how Syag holds, but others are definitely saying to keep adding.
The BT question is still legitimate. Even one a BT starts observing the mitzvos, it is still much more likely that they will forget to light candles than someone who has done it habitually. I don’t believe the custom to light a candle per child is observed alongside that to add for forgetting, I think people either hold one way or the other (I could be wrong).