Answer to #1
The web sites where you can make the booking, preferably DB (the German Bahn), CD (the Czech Railway) or ÖBB (the Austrian Railway) have the correct information.
The Eurail app is only uppdaterad on a regular basis and is not constantly on line against the different railway companies.
You can often buy reservations via the operator of the train and often you can choose from a seat plan or give your preferences. Via Eurail or the app you pay way to much for the reservation without the choosing service.
P.S. for your trip Salzburg to Prague seat reservations are only optional to be sure 100% to get a seat.
There are more ways to get from Salzburg to Prague. RJX to Linz and than change to EC to Prague (almost every 2 hours), or RJX till Vienna and change there for a RJ to Prague, or via Germany.
For THIS route you do not need REServ-its optional.
There MAY be works along some part of the route (its rather long+slow) and these are not in the app, as it does not get updated too often It should be self-evident that local sites of railways concened are always best..
As I was in Praha a week or 2 ago: there then were certainly works (sprava zeleznice) along the tracks south of Praha.