Should I buy Eurail pass first then book seats? Where do I reserve seats?

Yes that's the way to go on Rail Europe. Upper deck is more fun and better for the views. :)

Table option if you want to sit together.

Btw if you want to save money you could do this easy workaround. Direct TGVs to Geneva have a 39€ reservation fee in 1st class, however if you alight at Bellegarde (the previous stop, right before the border) it costs 10-20€ per person. It's then a short hop on a regional train across the border -> all in all it takes about 30 min longer.

Yes there will be a bus from Domodossola to Milan. Expect a 1h-1h30 longer journey than usual.

Easier (and probably cheaper) to book a 1st class pass in the first place, especially with the sale.

Upgrades are possible in Switzerland but they're not cheap.

Yes you can sit in 2nd class but why would you? :) The reservation fee is 10€ cheaper on the Paris - Geneva TGV but otherwise no difference.

I guarantee you'll be grateful for 1st class during peak season : bigger seats, less busy,...

This train does also go through Domodossolo (means it will not run). (Going that way would mean going from Interlaken via Spiez to Bern and directly back to Spiez so doesn’t make any sense.)

But there’s no need to avoid this. There’s a frequent direct bus service between Domodossola and Milano (2h 20min; that’s about 55min more than by train).

Some timetables for August are already published. Depends on train type and route. You do already see the replacement busses between Domodossloa and Milano if this is your question. Missing trains will appear step by step.

Timetable for August 6th (already published):

18.30 Interlaken Ost

18.52 Spiez

Change trains

19.05 Spiez

20.12 Domodossola 

Change from train to bus

20.30 Domodossola

22.50 Milano Centrale 

Use the companies planners. DB Navigator app and bahn.com are reliable tools of the German Railways but still dependant on the companies sending their data to the European database.

For now you can check Interlaken Ost - Domodossola on SBB app/sbb.ch and Domodossola - Milano Centrale on trenitalia.com.

In a week or two the buses should appear everywhere. There's no rush, don't worry.

The replacement buses seem to require reservations. There is no assigned seating so it's probably to make sure that everyone has a seat. They should be available through Rail Europe or ÖBB. In any case a regular ticket for it costs 10€ while a regular train ticket is 25€ (and a passholder reservation 10-13€).

Rail Planner shouldn't be used to plan. It is unreliable and less frequently updated. I only use it to log trains.

That's possible (but it's more close to 4 hours by daytime bus). But these would be busses not covered by your railpass so you need to pay for it.

But why should you want to go by bus if you are holding a railpass?

Maybe you are not aware of being in a different part of Switzerland now and going into Italy a completely different way? From Luzern to Milano it's possible to go by train at least every hour (direct connections do require a reservation; personally I'd avoid them and change in Lugano to a regional train).

You're too early. It looks like there will be engineering work and timetables are noot final yet.

Italian day trains don't often sell out and it they do, it's around busy days (begin/end of holidays etc.) and then still only a few days in advance. Just keep trying regularly.

Swiss trains cannot (and shouldn't) be booked. Simply hop on and sit wherever you want, the train ends are always less busy.

The bus is unlikely to sell out in the evening, maybe book a few days early in case. A ticket costs 10€ (no idea with passes, reservations seem to be required).

for the prestige class you need on top of the 1st class pass a reservation and upgrade bookable via official website of Goldenpass Express.