"Unfortunately, the page doesn't exist."
OK, I don't know why they changed the URL, but I found a schematic of the Bavarian track system here. I'm pretty sure it's the same map I linked to before.
As for the train to Füssen, they are working on that line somewhere. The Bahn schedule website shows it. For now, until October 30, there is a SEV (Schienen Ersatz Verkehr), replacement bus, from Marktoberdorf to Füssen. Marktoberdorf is normally the last station before Füssen on that lline. The bus should be waiting next to the station. People will get off the train and head to it The scheduled time between the train arriving in Marktoverdorf and the bus leaving is four minutes, but if you are trying to get to the bus, I'm sure they won't leave without you. Just don't stop for a coffee in the station when you get there.
"We had planned to take the train from Fussen thru Austria and into Garmisch."
The construction work at Füssen Bahnhof should not have mattered. There is not a direct rail connection from Füssen to Austria anyway, but there are numerous bus connections. The shortest connection (27 min) would be a bus from Füssen to Reutte (which is in Austria). There might be more bus connections to Pfronten, Germany, which is also on the Außerfernbahn rail line to Garmisch-Partenkirchen (via Reutte). Finally, there are direct and one-change, albiet longer, bus connections from Füssen to Garmisch-Partenkirchen via Oberammergau.