Validitity of Bayern ticket

I am slightly confused by your phrase "I bought a Bayern ticket . . . . for a return trip from Fussen to Munich on 19 Dec at 406pm by mistake."

A Bayern ticket (like all the Länder tickets) is valid from 09:00 until 03:00 on the following day. You do not buy it for a specific train, you buy it for a date. It is valid on any local train or bus (but not IC or ICE trains).
I am also confused why you have "bought" one already for a date in December. There is no advantage in buying in advance, you get exactly the same price if you buy on the day from a ticket machine at the station.

A trick if your train departs just before 09:00 (say 08:55) is to buy a normal ticket to the first stop the train makes after 09:00, then use your Bayern ticket is valid from there.

Information about validity here: https://www.bahn.com/en/view/offers/regional/regional-day-tickets.shtml?dbkanal_007=L04_S02_D002_KIN0060_REGIO-INDEX-LAENDERTICKETS-OBEN_LZ01
A map of all train lines the Bayern ticket is valid on is here: https://www.bahn.de/p/view/mdb/pv/deutschland_erleben/allgemein/tickets/pdfs_laender-tickets/2015/mdb_190311_streckenkarte_bay_2015.pdf

I bought the ticket online via DB website where the start point, date and time of the travel needs to be indicated.. No advantage but I still bought it in advance.

I actually bought a outbound flexi bayern ticket from Munich to Fussen online forgetting that I can return to Munch with the same ticket. So, I bought another return saver ticket from Fussen online on the same day at 406pm in the afternoon.

I have since cancelled the outbound flexi ticket from Munich. As the saver return ticket is not cancelable, I am checking if I can use it for the outbound journery from Munich too after 9am.

"As the saver return ticket is not cancelable, I am checking if I can use it for the outbound journery from Munich too after 9am."

If this ticket is a BAYERN TICKET, then the answer is yes. Chris wrote, "You do not buy it for a specific train, you buy it for a date," but this is in error because in fact you bought it for the specific train trip at 4:06 from Füssen, and your ticket shows that specific journey. Nonetheless, if it also says "Bayern Ticket", it is valid anywhere within Bavaria on that date from 9:00.

I suggest you refrain from purchasing any further tickets online without checking in here on the details.
"My tickets to enter Neuschwanstein Castle is at 150pm. Do you think I can make it?" Are you aware that the Bayern Ticket also covers the bus ride to the Hohenschwangau bus stop - and back? Enter Munich for your start station and "Hohenschwangau Neuschwanstein Castles, Schwangau" as your destination and you will see the connections.

Yes, it is indicated at Bayern ticket and it cost Euro 31 for 2A and 1C under 6 years old.

The reason I bought online is I am worried I may have a language barrier at the ticket machine in Munich HBF.

Does that mean I do not need to buy a return Bayern ticket fr Munich to Salzburg in advance too even though it will be over Christmas period?

Is December, winter holidays. I am worried the trains will be full..

Thanks!

"Does that mean I do not need to buy a return Bayern ticket fr Munich to Salzburg in advance too even though it will be over Christmas period?"

It does not matter what the season is. Your current ticket is valid for a full day of travel - December 19, from 9:00 onward. You can travel Munich-Neuschwanstein AND Neuschwanstein-Munich on that same day. (In fact, you could take 10 or more different trips within Bavaria on that same day if you wished to and had the time, as long as you use the local/regional train (or buses.) No additional ticket is necessary. It is the same for Munich-Salzburg-Munich... one ticket for one day, as much train travel as you wish (after 9 am weekdays, however, unless the weekday is a holiday.) (Note that the Bayern Ticket is not valid on buses in Salzburg, however - just for train transport between Germany and Salzburg.) And it does not matter how busy the trains are either... You can always buy a Bayern ticket at the station from a ticket machin (choose the English option.) If it's so busy that you cannot find a seat, your ticket allows you to stand.

The ticket machines start in German but have a very clear indication (multiple national flags) of the way to change the display to English and several other languages, so you would be Ok.

The Bayern Ticket is only valid on trains with no seat reservations, no matter how crowded or not if you fit you can get on, and it gets you no special privileges over any other traveler. For example it does not guarantee you to get on if it is overfull, you will have to wait for a following train. I've never seen it that crowded.

Just to clarify -- for instructions in English, look for the British flag (Union Jack) because you will NOT see an American flag on the ticket machines. Or on railroad websites.

As the saver return ticket is not cancelable,

As Russ pointed out, I'm sure you meant a Bayern-Ticket. A "saver ticket", or Savings Fare ticket, or, in German, a Sparpreis Ticket is an advance purchase ticket for a connection that includes at least one leg on a long distance train (ICE/IC/EC), which would be difficult to do for Munich to Füssen, as the direct route is by regional train(s). A Savings Fare ticket is cancellable with a penalty.

Aside from using the Bahn Ticket Shop, which I can't find anymore, and was only in German anyway, the only way to purchase a Bayern-Ticket is to search for a route for which the Bayern-Ticket would be a price effective ticket, and then purchase it for that route. But that doesn't mean that you must use it, or can only use it, on that route - it's just the only way you can purchase it. As mentioned, it's a pass, valid all day (after 9 work days), anywhere on regional trains in Bavaria. You can use it for the trip out, as well.

As for purchasing the Bayern-Ticket in advance, there is no price advantage either way, but you avoid the worry about a line at the ticket machine that morning. However, the ticket is date specific; you can't change your mind about the date.

By the way, the train gets to Füssen Bahnhof at 11:55, but you are not done traveling yet. You still have an 8 minute (12:05 to 12:13) bus ride from Füssen Bahnhof to the bus station in the village of Hohenschwangau. You catch the bus at the stop in front of the station and the Bayern-Ticket covers that bus in addition to the train. It's a few minute's walk from the bus stop up the hill to the ticket kiosk. You are required to pick up you ticket one hour before the tour, but it won't take very long to get to the kiosk. After picking up your ticket, you can walk to the castle. A fairly healthy adult can do it in less than an hour; there are also horse carriage to take you up the hill (6€/adult), but there can be a substantial wait for them.

If you want to leave on the 8:53 train and get to the castle kiosk an hour earlier, you can purchase additional single trip tickets from München Hbf to München Pasing (zone 1) for 2,80€/adult. The single trip tickets are valid until the train gets to Pasing at 8:59. When it leaves Pasing at 9:00, the Bayern-Ticket is valid.