I think the second is never going to happen without a desire to lose the next presidential election.
If they think having Trump in charge is worse than a Democrat they will do it. If they think Trump will definitely lose they will do it. If they think they can win without Trump, they will do it.
All the above does require a pretty strong case that Trump has done something illegal.
The problem for the first is that they need the same watertight case. There are currently no smoking guns.
They need a strong case, it doesn't need to be completely watertight. In fact, if the Democrats have a strong case, they will have to impeach, or be accused of putting politics ahead of justice.
However, I agree that there is currently no smoking gun. Lots of Americans seem pretty optimistic about the Mueller inquiry, but I think that is wishful thinking, there is no evidence that Mueller has got a case against Trump himself at the moment.