Let me answer your questions.

There is no quota for passholders on the vast majority of trains, exceptions are Eurostar, Thalys and TGVs to Belgium. But once a train requiring seat reservations is full, it's full.

With a pass you have a lot of flexibility as you can basically choose 2 min before boarding where you'd like to go.

You have chosen fantastic countries for the pass : Switzerland does not require seat reservations at all, Germany has optional ones (a good idea on long-distance journeys, they do not cost much) and Italy has a very good network (even if high speed trains require seat reservations there, there will be availability the day before on most trains).

Germany : 3€ per train or 4.50€ per journey

Italy : 13€ Frecciarossa, 3€ IC

There are always reservation-free alternatives, just in case. It's best to avoid making reservations on eurail.com due to extra fees and more expensive optional reservations (8€ instead of 3€)...

The Eurail Pass generally does not include transportation in cities, exceptions are suburban trains (but not worth to use a pass day for them, buy regular tickets instead).

Yes it is possible to book seats together. Please avoid eurail.com to book, see above.

Btw a 1st class pass doesn't cost much more, if you'd like to. :)

You already have tickets for the trickiest train, Eurostar (France-England-Belgium). You could have reserved seats for that train, but as tickets are not-refundable, you’lld better stick to your ordinary tickets.

Your pass = ticket. And it’s valid on more than 90% of all trains in Europe. For some trains, you’ll need a seat reservation.

Wether to reserve seats in advance, really depends from train to train. Which trains do you plan to take?

There are three kind of trains:

• Reservation free trains (Most trains in Europe, mostly local and regional services. S, L, IR, RE, RB, Sprinter, R, TER,… many different names)
• Trains with optional reservations (Long distance trains in Germany and the UK, IC, EC, ICE, RJ,...). You can board any train you’lld like and find a free seat, but if someone shows up with a reservation you’ll need to find another seat. Advisable to reserve on busy travel days, like summer-weekends, long holiday-weekends,…
• Trains with mandatory seat reservations (TGV in France, Eurostar, Frecce-trains in Italy). Without a seat reservation, you don’t get on it. These trains can sell out, sometimes weeks in advance (Eurostar).

If you plan to visit multiple places in France, it’s best to reserve your TGV’s in advance (If you’lld like to go to Bordeaux, Lyon, Lille, Strasbourg,...). You can do that here: https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish , but you’ll need a pass cover number (generate it here). As these trains can sell out quite easily in the busy summer holidays.

For all the rest of the countries you’ll be okay. Trains have no reservations (Belgium, Netherlands), optional reservations (Germany) or mandatory ones (but that don’t sell out that quickly) in Italy. Just in France I would book seats in advance.

If you reserve seats for two people, you’ll be sitting always together. No need to reserve them seperatly.

Avoid Thalys trains between Belgium and Germany. It’s seat reservations are just too expensive. Take the ICE in Brussels (two-hourly) to Germany. Go to coach 22 of the ICE in Bruxelles-Midi/Brussel-Zuid if you don’t want to reserve a seat, for one reason or another this coach doesn’t have much reservations in it. Although I’lld recommend to get a seat reservation here, since these trains are very popular in Summer.

There’s a good community guide on how to get reservations here (without much extra cost).

There are always ways to avoid seat reservations, just ask if you want to know for a specific route.

Local travel is usually not included (Bus, tram, Metro,...). Local trains are (like the S-bahn in Germany or the Elizabeth line, or the London Overground in London, these are included). RER in Paris however is not included.