All great questions.I will try and answer.
1) This La Nina would be considered a moderate strength event with an anomaly of around 1 degree Celsius below average in the central/eastern equatorial Pacific.
2) It currently is weakening and is expected to continue to weaken through the end of winter and into spring. 

3) One reason for these atmospheric river events is that the jet stream has been extended across the Pacific pushing these events into California. This happens from time to time in winter although it tends to happen less often during La Nina winters.

4) Impacts are normally observed more in the seasonal totals as opposed to individual events. And they also tend to occur most often during winter. So it's hard to say if a region sees an impact earlier or later than others, as opposed to just in the season itself.