Summer timetables have not been published in a few countries yet, mainly thinking of France and Italy on your itinerary. You should check current times and expect pretty much the same (with a few caveats, see below).

Here is a good guide for seat reservations : https://www.seat61.com/interrail-and-eurail-reservations.htm

Some advice in chronological order :

London - Amsterdam : 35€ direct Eurostar with passholder quota, unavoidable (book ASAP, especially if your dates are set). For more options Eurostar to Brussels (30€) + hourly reservation-free IC train.

Amsterdam - Brussels : either 27€ Eurostar with passholder quota (avoid) or hourly reservation-free direct train. The latter generally takes 45 min longer but there should be faster trains starting this summer.

Brussels - Paris : 27€ direct Eurostar or cheaper/free but slower routes via Lille or Maubeuge.

Note that you'll be in Paris right during the Olympics. Expect crazy prices, more crowds and extra security measures. Wouldn't necessarily recommend.

Paris - Switzerland : 29€ fee for the direct TGV Lyria. 10-20€ for a domestic TGV to Strasbourg or Mulhouse for example, only 30 min longer. Timetables not published yet as said above.

Within Switzerland Eurail is very easy to use and good value. Seat reservations aren't a thing. 25-50% discount on mountain railways (Jungfraujoch, Titlis) but nothing above Zermatt.

The Domodossola - Milan railway (fastest route from Zermatt to Venice) is closed from 9th June to 8th September. Rail replacement buses will run as Hektor said in your other post. Plan with 1h-1h30 longer than usual, timetables not known yet.

It is not possible to travel overnight unless you spend hours outside in a random station at 2am (totally not recommended for a honeymoon!).

You've got too much planned in Italy. Cross Milan out of the list right away, it's not worth your limited time. Note that Italy will be crazy hot and crowded as it is every summer.

Separate post coming.

About your specific questions :

1. You only can know how many pass days might be useful. I'd probably go with 10 or 15 days as you intend to move around a lot. Using a pass day in Switzerland, even for short journeys, is good value.

Btw boats on Lake Thun and Brienz are fully included (by using a travel day). Fun experience!

1st class is definitely worth the little extra cost, especially during peak season.

2. Use the link I posted above.

3. Speak to staff as soon as it's likely you miss the connection. They'll give you a proof of delay and/or issue new reservations for the following train if they can (rare). Availability on some international routes can be limited so plan with margin in the first place to avoid getting stuck.

I'm not going into details.

If it's a train with optional/no reservations, it's easier : simply log the new one on the Rail Planner app (as usual) and hop on. Afterwards claim compensation if you arrive more than 1h late at your destination.

4. The pass is not valid on subways, trams or buses, nor on some suburban trains (Paris RER network). Such city tickets are rather cheap in the first place. Except in Paris where prices will double for the Olympics…

5. Use SBB app for Swiss travels : very reliable and useful. As I said timetables not published for Italian trains yet nor for works towards Milan.

Last helpful link : https://www.seat61.com/how-to-use-a-eurail-pass.htm

Thanks for such a wonderful response.

Could I ask a couple of follow-up questions please?

When you said “Btw boats on Lake Thun and Brienz are fully included (by using a travel day)”, do you mean cable cars and funicular are also fully covered by Eurail Global Pass?

We are keen to do a Mt Pilaturs Golden Round Trip when in Lucerne. 
Lake Thun alongside Niederhorn from Interlaken.

Wondering if it is only 50% off on Global Pass? Should we buy Swiss Travel Pass to be more efficient? What would the best option be?

Thank you,

No, when I mean boats I mean boats. They are run by BLS (a railway company) so that's the explanation.

With Eurail you get 25-50% discount on other lakes and on many mountain railways. Have a look here :

https://benefitsportal.eurail.com/benefits/swiss-private-trains-benefits/

https://benefitsportal.eurail.com/benefits/swiss-private-boat-benefits/

The Swiss Travel Pass isn't valid everywhere either, it gives you a 25-50% discount where it isn't.

It's up to you to make calculations : list prices with/without passes and then decide. Regular tickets from/to Paris will be very expensive so that's why I'd get Eurail.