1. You can check occupation on the train operators website. Seat reservations are a good idea on busy mainlines such as Munich - Brussels. It is recommend to avoid eurail.com for seat reservations (2€ booking fee per person per train). Use other websites (less or no fee), have a look at :

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

https://www.seat61.com/interrail-and-eurail-reservations.htm

2. If your train is delayed, often the connection is guaranteed for a few minutes. If you're likely to miss a connecting train with mandatory reservations, speak to staff on the train and they'll redirect you to the next train (free of charge). When no reservations are required, simply take the next one. Trains often run hourly, if not more so you can always decide to wait a little bit at the station if the connection is too tight for your liking.

3. Seat reservations cost the same for children.

4. It depends but you shouldn't wait for those trains, esp. Thalys between Paris and Antwerp as it has a limited quota (can sell out weeks in advance). There are always alternatives though. You need to book night trains ASAP if you're planning on taking them.

5. You add any train you'd like to the app before boarding. You then get a QR-code which is your ticket. You can do that at any point prior to boarding.

6. Seat reservations are usually fine on PDF. They'll never appear on the app. Whenever a ticket inspector comes you show both the PDF and the app (for the pass).

7-8. Explained in my last post.

9. There usually is a waiting space (seats) or what do you mean ? In big stations there are lockers for luggage too.

10. Rail replacement buses when there are engineering works + boats on Lake Thun and Brienz (Switzerland). Those are mostly exceptions. You get a 25-50% discount for most mountain railways in Switzerland.

11. With a 1st class pass you get free optional seat reservations in Germany, Austria and a lot of other countries through tickets.oebb.at

12. No

13. If your itinerary isn't fully set, I'd travel in this order : France - Belgium - Switzerland - Italy - daytime or night train to Munich - Munich - Berlin + night train Berlin - Copenhagen. As long as you board the last train before midnight, only one pass day is used.

Can you please explain what you mean by booking the Night Trains ASAP? What is the benefit of these, as I found them quite long, for example, 10-16 hours?

Sorry, couldn’t get you Can you guide where I can find this answer?

Yes I mean to keep my 2 x luggage bags (23 KG each) temporarily for 1-3 hours, while I can explore that city a little bit (or maybe take a few photos)

• I read on the Eurail website that Bernina and Glacier Express are totally free with Eurail Global Pass! (?)
• Boats on Lake Thun and Brienz are included with this pass (for free)?

Unfortunately, my itinerary is set & I would not be able to make major changes due to hotel bookings (though it is far from perfect ).

Why are you suggesting night trains within Germany? Are there any advantages? And by ‘night trains’ you mean the trains showing me “+1” with them?

With a nightrain you can get from one city to the other and have an occupation. And less than 8 hours a night train make no sense, because you will have less sleep time. Night train in Europe are very busy and get full booked soon. 

See answer 2) of Thibcabe

 If your train is delayed, often the connection is guaranteed for a few minutes. If you're likely to miss a connecting train with mandatory reservations, speak to staff on the train and they'll redirect you to the next train (free of charge). When no reservations are required, simply take the next one. Trains often run hourly, if not more so you can always decide to wait a little bit at the station if the connection is too tight for your liking.

If you have an delay over 1 hour you can ask for delay compensation via Eurail. For all other issues (like last train of the day missed, to get a hotel instead or taxi) you need to get in touch with the train company and staff at the station. https://www.eurail.com/en/help/delay-compensation

8. For trains that are showing “+1 day” (arriving at the destination on the next day), will this deduct 2 days off my Global Pass?

It only says that you will arrive the next day. 

You need 1 day if you do not change trains after midnight. You need 2 days if you use a new train after midnight. 

Yes there are, but check for every station before via Google. In Italy often there are no luggage storage due to anti terror rules. 

Yes the transportation, but not the reservation is free.

On Bernina Express you can also sit on the front of the train in the carriages that have no panoramic windows, but instead you can open it. In the panoramic carriages you need a reservation. 

Yes boats of BLS on Lake Thun and Brienz are included. Add the boat to the pass and you are good to go. 

The advantage is that you can sleep and get from one city to the other without needing a hotel. A night train has more confort classes. Seats, couchettes and sleeping coaches. Some people love it, other like it for the service. Other don’t like it because noisy and small (it is a train).