Does anyone know the mechanism by which she could be forced to withdraw? Impeachment is usually for breaking the law, not putting forth a bad one...
The bill is not in the legislature yet, so it is up to her whether the bill gets brought to a vote. It sounds like even pro-Beijing lawmakers are growing weary of this bill, saying that this isn't worth the fight that's being made over it. The Article 23 National Security law is more of a priority for a lot of them, but that was similarly unpopular the last time it was brought up.
The most likely outcome with Lam leaving the position of Chief Executive is for her to resign. This could be on her own accord, because her legislation has proved so unpopular, or because of pressure from Beijing. This is not making Xi look very good--both internally and internationally. One of the most consistent demands of the protesters right now is for her to step down. It would not be surprising for her to do so if these protests continue.