Ljubljana to Munich reservation or not

3.) The 6 minute transfer is possible if the train is on time. If you miss it just take the next one. The connection is arround once an hour (some of them require an additional change) as there are no mandatory reservations on that route taking the next train is no problem. 

In that direction its not that bad to miss the connection (happened to me in the other direction and the train to Ljubjana runs less frequently)

1.) The reservation is optional so its up to you, if you already know which connection you will take purchasing the reservation might be a good idea(at only 3€ its not that expensive) I took that train (in case your connection involves the ICE) on a very busy day(long weekend) from Graz to villach after my orginally planned train was way too full(it was headed to italy) and there were still unreserved seats. Its up to you.

On Sep 13th trains might be busy in Austria as it is the last day of school holidays in the western provinces. So far ÖBB has not given the “please reserve seats” advice for the Villach-Munich trains. I would buy seat reservations anyway and select the seats from the seat map, which is possible on ÖBB railjet trains. 

6 minutes connection time should be enough. And as Marvin has said, you may take the next train if you miss your cannection. The ticket office should be able to exchange your seat reservation for the connecting train. But I would not take the risk missing the connection RG/D 312 to RJ 110 at 17:22/17:28 in Villach as it would become a quite late arrival in Munich.

Thanks for that;  I didn't realise the schools returned so late 

Right, so I am trying to book the reservation.  Just to confirm, does the message on the Ljubljana to Villach train mean it is not possible to actually book a reservation, or us there actually no express train 314 running?

The 13:30 train runs but doesn't have reservations.