1.dont worry, down yundah-it will all come OK. Plenty of trains and seats.

2.on 11/12 is the main EUR timetable change-which mostly does not say that much. But it means that railways are/were uncertain to exact timings and then refuse to release bookings, as they do not want people to tell later it may have changed by 5 mins.

3.it is very well possible to do this whole route on local/regional trains without any RES_but thats slower and wants more changes.

4.the train FROm Zm is RES-free till BRIG. Then you can-you choose- use the direct EC to Milano-or locals via Domodossola (first stop in IT)-change to other TreNord local. EC is 11€ for pass-run ev 1 - 2 jhrs or so-so plenty of choice, if you are not too upset about perhaps being an hour earlier or later. This is not NSW-rail with just 1 train/day to WaggaWagga beyond the back of Bourke. These RES can also be done at any Swiss counter for same price and they will be very well used to doing that and explaining.

5.From MIl to Vnz are hourly frecce=superfast treni by TrenItalia, must RES (10€) OR also hourly local TreNord till Verona-change other regional to Vnz. TrenIT is always, always notably very slow with new data and again there is no need at all to worry weeks ahead. Can online RES via oebb.at-as in the linked overveiw anna B fra Sverige always greets newbees with.

However, much higher demand is usually on the days just before and after Xmas-obviously and on Xmas days service may be reduced-but trains still run here.

If you mean reservations instead of tickets, then the best place to book for Switzerland to Italy is via tickets.oebb.at. Brig to Milan is €11. DO NOT use "Seat only” but add the Interrail/Eurail discount and then look for normal tickets. If that doesn't work for you, then please let us know date and departure time.