1. Yes. When placing a signal, you put it on the right hand side of the track from the direction you want the train to travel in on that track.
2. If it is on the left hand side then it is facing the other direction yes.
3. It’s a good idea in general, though it would depend on the specific setup you have (sometimes you want a train to wait further back). But if you had no signals on a track with 2 stations and 2 rails most of the way, with a short single track area and 2 lines running on it with 1 train each, then the moment one train left a station it would reserve the track all the way to the other station and the other train would be stuck at it’s station until the first train had gotten past the single area. Putting signals on the incoming tracks just before a merge means that trains will only reserve the track up until the signal and other trains can still use the track. When the train reaches the signal, then it will reserve the track again (or wait if there’s another train already there).