Deutsche Bahn Website

My interpretation is that it doesn't run the exact same schedule every day, like it won't run at the same times on 18 Nov., but it does run the same schedule on the 19th of Nov through the 11 of December, but then not on Nov 20-25. If it shows the train on the schedule for the 26, the train should run that day. Change the schedule to Nov 18th, that train that train leaves at 9:22. What do you mean a "railpass" for Nov 26? Do you really mean a Bayern-Ticket? That's an all day "pass" for regional trains in Bavaria. It cost €28 for up to five people. With that "pass" you could take the 9:42 RE to Salzburg. The other two trains are express trains (RJ & EC); you couldn't use them. If you wanted to get to Salzburg an hour earlier (10:42 vs 11:42), you could take the RE that leaves the Hbf at 8:48. That train gets to Grafing Bahnhof after 9 AM, so MVV 3-zone single trip tickets at €7,20 ea would cover you travel to Grafing. The Bayern-Ticket would cover you from there. Note also: if you are doing this as a RT day trip, you can use a Freizeit Ticket (page in German only), €30 round trip, pP. With the Freizeit Ticket, you can leave as early as you want. It is valid on Bahn express trains (EC or IC, not regional trains, not RailJet). There is no advance purchase requirement. You have until 3 AM the following morning to complete your travel.

I think that means it is 20 November to 25 November that it doesn't run. But all three trains show that they will run on 26 November so I think you're good to go. Sorry, I can't help with the car question (I have driven to Salzburg once, but that was many years ago). Driving in Germany is generally quite easy and smooth, but parking may be a problem.

After reviewing the schedule, if you are looking at the 9:42 RE, for the period Nov 19 through Dec 11, it runs on the same schedule from the Hbf at 9:42. However, from Dec 20-25, they are doing work on the tracks between Grafing Bhf and Ostermünchen, so the RE starts in Ostermünchen at 10:17. This won't apply to you if you go on the 26th, but it would a day earlier. DB suggests you take the EC from the Hbf at 9:31 and catch the RE in Rosenheim, but, of course, you could not do that on a Bayern-Ticket. However, there is an RE (88833) leaving the Hbf at 9:12, getting to Grafing Bhf at 9:35. There you will be met by a substitute bus (SEV) which will take you to Ostermünchen where you can catch the RE to Salzburg. You can still do all of this with a Bayern-Ticket. Incidentally, depending on where you are staying in Munich, you might find it easier to catch RE 88833 at 9:19 at Ostbahnhof. As for using the Bahn website, this is kind of subtle. When I looked up the schedule for Nov 25, it showed using an EC (85 to Bologna) to Rosenheim and the RE from there. I looked at the detailed schedule for the RE and it showed it starting that day in Ostermünchen. So I put "Ostermuenchen" in the via. Since the EC doesn't stop in Ostermuenchen, it stopped showing the EC and showed, instead, the RE to Grafing and the SEV from Grafing to Ostermünchen. I could have accomplished the same thing by specifying "only local transport", which eliminates train not covered by the Bayern-Ticket.

I see that the 10:27 EC is still bookable online for €29 for both of you, one way. That's with a non-refundable Sparpreis ticket. The 9:27 RailJet was €29, but the €29 tickets are sold out; it's still possible to get one way on that train for €39 for both of you.