Sorry for the confusion. Adding the train to the app can be done at any point. It works like a diary.

However you need to make seat reservations in advance when required : through tickets.oebb.at for this route and all Italian trains.

The transfer in Brig is simple :

- arrival in front of the station

- enter the main building on your left

- go up the stairs or the ramp to plaftorm 4

I've done it a few times, although not with luggage (it shouldn't change much). I'd definitely do it. If you're worried, take the 07:37 train.

Thank you very much.
1.

In case I miss the planned train, which includes a paid seat ticket from Domodosola to Milan, can I take the next train, or should I buy another ticket?

2.
Can I cancel or change the ticket that I buy on OBB with Eurail discount if my schedule changes?

Any fees? If any, please let me know how much.

Thank you in advance.

1. If you miss it due to delays, speak to staff at Brig train station and they'll issue a new one free of charge. In addition you can always take reservation-free regional trains.

2. Seat reservations are generally non-refundable.

Thank you very much.

If I miss the train not due to the train delay but my fault (such as being late for transferring etc,), then will the the staff also issue a new one free of charge for the next train? Or else what should I do in that case?
 

From Milano (Italy) to Venezia, the following schedule is listed on Eurail.com.
However, those schedule was not found on OBB.
Why is that?
I’d like to make a reservation on OBB. What can I do?
 

For which date is this?

It could be that the ÖBB booking website hasn't been updated yet with the latest timetables.

It’s on 9, Aug., 
Then, should I wait until OBB is updated?

By the way, if I miss the train, for which I reserved a seat with Eurail discount, not due to the train delay but my fault (such as being late for transferring etc,), then will the the staff also issue a new one free of charge for the next train? Or else what should I do in that case?

That's indeed my suggestion. Those trains won't be fully booked m,ore than a day (or so) in advance, if ever.

If you miss your booked train by your fault, then normally you'll have to buy a new reservation.

Thank you.
In case I have to buy a new reservation because I missed my booked train by my fault, can I do it right at the train station? Would the tickets from Brig to Milano (actually tickets from Domodosola to Milano) be not fully booked on 9, Aug., on the day or a day before in general? 

Yes you can.

My crystal ball doesn't work very well :)

But it's in the middle of the week and there's no Italian or Swiss national holiday so I see no reason why it would be very busy.

There may or many not be seats left on the ECs from Brig on the day : the trains aren't that big and they can be pretty crowded in the summer. If you're alone, it shouldn't be a problem. 3 people (which I assume you'll be) shouldn't be an issue either but you'll likely not sit together.

There are always reservation-free regional trains available so if you miss the EC, this is the way to go.

- IR Brig - Domodossola 10:44 - 11:12

Then small stop in Domodossola, a nice town for a coffee :)

- R Domodossola - Milano Porta Garibaldi 12:01 - 14:14

- R Domodossola - Milano Centrale 12:56 - 14:35

The next EC is 3h later.

To Venice : the fast trains mainly leave from Centrale but there's one at 16:13 from Porta Garibaldi for example.

But don't worry, you surely won't miss the train anyway... :)