A potentially great idea. I know I would have loved to do this trip as a 5 year old and still today
!! What a lucky child and wonderful parent!!!
In response to your questions,
1. Not 100% sure as I am not from that area, but I have ridden the route at least 3 times. But be aware if on time most of the Cascades will pass in the dark-- indeed most of your trip will be in the dark according the current schedule (which is the summer schedule) The Cascades will be the area you are most likely to see snow. Indeed, From what I know about the area and the weather, the only section you are likely to see snow is between around Dunsmuir CA and Chemault Or, which corresponds to between 4.58 am and 8.59am (Website for timetable is chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.railpassengers.org/site/assets/files/20928/coast-starlight.pdf) Remember that you are going when daylight is near its shortest.
2. Delays could be anywhere from none to hours. The times I took the train there was between less than hour delay to four or 5 hours. If long delays, you are more likely to see the best scenery at a civilized hour (the Cascades) and thus snow.
3. You probably know this, but you definitely want to have plenty activities and snacks you know your 5 year old likes as well as healthy snacks. The snacks available on Amtrak are generally not very healthy For details check out the Amtrak website.
Finally, I am concerned that this might not be the best way to show you child snow for the first time. While it will be extremely cool for the minutes they first see snow and from a train, will they stay engaged enough for a pretty long trip? and the length has the potential for great variability. You of course know best.
Also how well will your child do sleeping in a new environment that is moving? Will this be different in a sleeper and a coach seat? Being in a sleeper if you never have been is pretty amazing but how well does your child calm down?