1. That’s entirely up to you. In most countries 1st class is quite a beacon of quietness and a bit more spacious. Sometimes larger tables as well. Might be worthwile if you have a studying son. But it’s quite expensive.
2. The pass is your ticket. For most of your connections (Barcelona-Marseille, Milan-Rome, Rome-Venice, Venice-Munich you probably need to reserve your seats. Seat reservations in 1st class cost more than 2nd. More information on reservations you find here: 

   PS: The Barcelona-Marseille high-speed train is very popular, and probably sold out already. There are only a limited number of pass-users allowed on that train. But you’ll have plenty of alternatives using local trains via Portbou-Cerbère.
   PPS: First class tickets in Germany are automatically sold with seat reservations, so you might want to reserve those as well.

3. Indeed, you never know. Seat reservations are best bought in advance.

4. For your routes, not really. You’ll miss out the Alpine scenery if you use it to go from Italy to the North.

5. Depends on your preferences; sometimes. But again, if you don’t want to miss out on the Alpine scenery, you might want to take local trains that follow the slow, curvy, winding, mostly spectacular route through the mountains. Some of the (higher) speed trains just cross the lot of the Alps under a long tunnel. Using Thalys with a pass, between Brussels and Rotterdam/Amsterdam is not worth it, but that’s not in your itinerary;

6. Indeed, everything is done via the app. Sometimes you want to print out the reservations to wave it in someone’s face is somebody’s sitting on your reserved seat. But a pdf is just fine.

Hi Brendan,

Thank you so much for your advice! I have just purchased the global passes. 

However, I am still slightly confused as to making a reservation on a rail company’s site. I only see the prices for the train tickets … how do I get to make a reservation on the rail company site instead of going through the Eurail app? When I search for a train on a rail company site, I fail to see where to make the reservation - all I see are the ticket prices for the respective journeys…

I will go to the reservation link you have given me, but they do not cover all the stations. 
 

If they are not specifically mentioned in the thread linked by Brendan then most likely a rail company website does not provide a facility to allow seat only or eurail passholder reservations to be booked with them.

Specifically, SNCF, RENFE and Trenitalia do not provide this on their websites.

These 3 train companies and their subsidiaries (Thalys, Eurostar) cause a majority of the issues seen here due to their blocking of direct passholder booking online and other anti-eurail policies. It is a known issue that seemingly nobody in charge is willing to tackle.

For your itinerary, as explained above the main issue you will have is a train from Barcelona.  The direct high-speed service from Barcelona to France is problematic on a number of levels and you should just forget about trying for it.

The alternative is to use the local service to Cerbere, full timetable here https://rodalies.gencat.cat/web/.content/02_Horaris/horaris/R11.pdf

then look for suitable connections to Marseille. Connections are available both with and without using high-speed TGV, journey time without is often not that much slower.

Sites like d bahn (germany) have a seat only option. OEBB (Austria and Italian trains) you add a discount code for each passenger of interrail/eurail pass, and for French TGV s use the b-europe dedicated site for pass holders. For others just ask if not clear in the link above.

Wow, thanks AI_G and Yorkie,

You really helped me understand the process a bit better. Let me see how far I get before another blonde moment strikes!

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations