Munich to Vienna

I just checked on the Deutsche Bahn website (they have the RJ and EC train schedules listed there too) as well and it looks like the train schedule is only available through December 10. I'm sure the same applies for the OEBB website. I would just check back in on another date when the schedules have been finalized.

It takes about 45 minutes to get from the airport to Munich Hbf on the S8 train, so if your flight arrives on time and you can make it through immigration and customs within an hour, you could still make it to the station on time. Although the train that leaves at 12:18, although it requires a connection, only takes 16 minutes longer than the direct, which isn't too much of a difference and it gives you more time to get to the station in case your plane is late.

The train timetables for most railway companies change every year on the second Sunday in December, that is 11th December this year. Most rail companies have not yet published their timetables past this date.
German Railways DB will open bookings for dates 11 Dec onwards on 18 October.
For more details, keep checking this webpage: http://seat61.com/news.htm

That said, 90-95% of the trains will be the same, or just altered by a few minutes. If there are 2 trains per hour currently, you can be 95%+ certain there will be 2 trains per hour after 11th December.
It sounds like ÖBB (Austrian Railways) are accepting bookings, with the small risk the times may change by a few minutes.

But, how are you arriving in Munich? If you are arriving by air, you cannot predict your arrival time. If you are arriving from outside Europe, your flight could be 1 hour late (or 30 minutes early) And getting through immigration (passport control) and out of the airport could take 30 minutes or over an hour. Then you need to get an S-Bahn train from the airport into Munich.
It is generally considered risky to book a non-changeable rail ticket after an incoming intercontinental flight.

Thanks for your responses.
@Chris: There's a train per hour on this route. So, as you suggest, they will likely keep it, maybe a few minutes here and there. OBB is definitely accepting reservations for this day. In which case, if the schedule changes, I assume I can still use the ticket I have as long as it is for the same hour/train... Maybe I can go with the 13:30 train, which will give me some room for adjustments.
I will be flying in from the US. The special advance booking fares are so low in comparison that the risk of advance booking seems worth it to be honest.

[...] Is it possible to make the 11:30am (direct RJ).
I want to make advance reservations because tickets are much cheaper that way. I know the risks involved.

If everything goes smooth, it's possible, but you can never be sure of that. But there's no need for gambling with discounted train tickets. Just travel on a Bayern ticket (always available, 23 E for one, 28 for two, valid after 9:00) to Salzburg and from there take the Westbahn to Vienna. Travel time is roughly the same, price is a bit lower than a discounted long distance ticket and much cheaper than a flex price ticket. So, for example

1. Buy a Bayern Ticket from a ticket machine in the central area ("Z")
2. Take the 10:24 S8 to München Ostbahnhof (don't take the S1, even if it displays "Ostbahnhof" as it's destination - it takes much longer)
3. At the Munich East station transfer to Meridian 79015, dep. 11:04, destination Salzburg, arr. 12:14
4. In Salzburg transfer to WB 919 (Westbahn, tickets available aboard, E. 25.50), dep. 12:52, arr. Wien Westbahnhof 15:20.

Total travel time: 4:56

Now compare the travel time of the S8 / RJ:

München Flughafen Terminal dep 10:33 S8
München Hbf arr 11:17

München Hbf dep 11:33 RJ 65

Wien Hbf arr 15:30

Total travel time: 4:57

Wow sla019, that's wonderful information that I will most certainly look into. Don't know what it would have taken to figure that out with any amount of confidence on my own! Thank you so much!
We are a family of three with a young child so I will just need to consider if we can easily navigate this. But there are risks with advance booking, in which case I must say this sounds like a great option.

Not sure why I am being allowed to make a reservation when the
schedule can change

Airlines do this all the time. Once they changed the schedule so that the first flight arrived at the connecting airport after the second flight departed. The airline didn't pick it up. I had to call them and inform them that I couldn't make that connection.

As for the Bayern-Ticket. You say your child is young. You only need a Bayern-Ticket for two at 28€. Your own children/grandchildren under 15 travel with you (up to two adults) for free with a Bayern-Ticket.

The RailJet goes non-stop from Munich Hbf to Salzburg and is therefore faster than the regional Meridian train, which makes multiple stops. However, you save time by changing at Ostbahnhof, so overall, it doesn't take much longer.