As to the usage of the app, did you check the mobile pass FAQ? The button to the right of each journey is to add it to My Pass, but you should only do that shortly before boarding a train, since it will also activate a travel day. If you then change plans and forget to cancel a travel day, then you loose it (or you have to go via customer support if you notice it).

The vast majority of trains don't require a reservation. The ones that do are usually long distance and/or high speed; it also depends a lot on the country and operator. E.g. in Germany, even long distance and high speeds trains can be used without reservations (except night trains).

As to what timetable you should believe: the app is one of the least trustworthy sources. It's offline and does not get enough updates to be reliable and therefore should not be used for planning. National planners or apps are more up to date (and more complete); for international planning, the German railways website or DB Navigator app is a good choice.

In any case, there's no direct train Amsterdam - London that leaves at 11:00.

As to your numbered questions:

1. That depends on the operator. Optional reservations usually aren’t, others can sometimes be refunded or exchanged, but often then only before the planned departure time.
2. The easiest way to book reservations depends on the exact trains you'd like to book. The app itself does not book reservations but forwards to (some) websites that can, including the Eurail website. I try to avoid it though because of its added booking costs. If you want more details, please specify exactly which trains you'd like to book (origin, destination, date, departure time).
3. The Amsterdam - London direct trains are only run by Eurostar. There are additional connections with a change in Brussels where you either use Thalys or an intercity train from Amsterdam to Brussels to connect with Eurostar there. To find trains, see my comment above about timetables to use for planning.
4. I suppose you mean reservations instead of tickets, since your Eurail pass is your ticket.
   Yes, some reservations are still only available on paper, most notoriously reservations to Spain and in Spain (but also some others). You can have them sent via snail mail (for which Eurail wants an extra 9€) or buy them at a ticket office. Also note that reservations are entirely separate and will not show up in the app.

Hi, thank you for the information you shared, it’s greatly appreciated.  The majority of my route says “seat reservation required” or “Pass not in Network”.  Am I doing something wrong or looking In the wrong place.  Here’s my intensity.  London, Amsterdam, Paris, Montreux, Lautterbrunnen, Munich, Salzberg, Prague, Vienna, Budapest, Venice, Rome, Sorrento, Almafi, Florence, Como, Nice, Madrid and Barcelona.  Am I better to book these now, some of the busier/overnight now, book them all at once as soon as I’m in London, etc.  Just easiest, most convenient process?  

2/5 London - Amsterdam
Reservation required (35€), you can do that via Eurail (2€ booking fee per seat).
But there's no availability anymore in the train leaving London at 11:04. The direct trains still have pass holder seats leaving London at 8:16 or 18:04.
The train at 11:04 does have availability to Brussels (30+2€); you could change there to the hourly reservation-free intercity.
Book Eurostar now, or at least don't wait until you're in London, since pass holder seats ofte sell out several days in advance and sometimes longer.

7/5 Amsterdam - Paris
Reservation required (€35), you can do that via Eurail (2€ booking fee per seat).
Also best booked in advance.

14/5 Paris - Montreux
The timetable in the app is outdated; it looks like there are engineering works in that weekend and some trains have been rescheduled. Please check German railways. You'll arrive in Montreux a bit later.
Reservation required for Paris - Genève (27€), you can do that via Eurail (2€ booking fee per seat).

17/5 Montreux - Lauterbrunnen
No reservations required, but you need an extra ticket Interlaken - Lauterbrunnen. Eurail gives 25% discount there, so it should be CHF 5.70. You can see to which train the remark applies if you open the connection. You can buy this ticket at any Swiss ticket office.
Note that the rail planner app, in its default settings, ignores changes shorter than 5 minutes. In this case, it can be done almost half an hour faster.

21/5 Lauterbrunnen - Munich
You'll need a ticket Lauterbrunnen - Interlaken with 25% discount.
No reservation required.

24/5 Munich - Salzburg
I can't see the train but normally no reservations required.

2/6 No journey?

Salzburg - Vienna does not require reservations.

5/6 Vienna - Budapest
The app timetable is outdated again. This train leaves at 11:42. In addition, reservations are not required. I'm not sure why it shows you have to change. My app shows no change and no reservations required.
Finally, Kelenföld is outside the centre in Budapest. Although it now has a metro to the city centre, you can also stay on the train until Budapest Keleti (final stop), which is closer to the city centre.

11/6 Budapest - Venice
You can also leave from Budapest Keleti at 18:40. If you want to go to the city centre, stay on the train until Venezia Santa Lucia (arr. 8:34).
No reservation required until Vienna; in Vienna, there's 9 minutes to change. You may want to leave Budapest an hour earlier (17:40).
For the night train Vienna - Venice, reservation is required, which is best done via ÖBB (using these steps, no booking fee). Seat €14, couchette €44, sleeper from €54 per person.
Availability seems good now but night trains can sell out; best to book a few weeks in advance.

I don't see any further trains in Italy or Spain.
Anyway, day trains in Italy can be booked via ÖBB the same way as the night train above, or at a ticket office in Germany, Austria or Italy. Pass holder seats are not quota-controlled and it's usually enough to do it a few days in advance.
The high speed train from France to Barcelona can be booked at a ticket office in Germany (or in France but not in Italy). It's best to book it more than a week in advance, e.g. as soon as you get to Munich, since there's only a limited number of pass holder seats. It's only available on paper, so ordering online means it has to be sent somewhere for additional costs.
Domestic trains in Spain can also be booked in Germany (but not in Italy, not sure about France), but pass holder seats are not quota-controlled, so booking one or a few days in advance should be OK.

I really appreciate your time and help!  Think once I get the hang of it I’ll be fine.  With that said, I don’t even see any of the times you referenced for London to Amsterdam.  I don’t even see a direct option.  I believe Im on the Eurail website so im completely confused at this point.  

It looks like the planner sticks 35 minutes footway before the Eurostar journey and claims you have to change:

The minimum check-in time is usually 30 minutes, although taking a bit more time is advised.

So what is the easiest, most convenient way to do this?  This is literally the first ticket I’ve ever booked, first trip to Europe.  I can see the schedules you mentioned directly on Eurostar’s website.  It’s pretty dang confusing/frustrating when every website is giving different information.  Even on the Eurostars website, I can see where I can book but it wants to charge me as if I don’t already have a Global Euro Pass?  

Unfortunately, Eurostar have removed the possibility to book pass fares on their website.

So I cannot book fares in advance with my Global Pass on their website, how am I suppose to book fares in advance?  

As I said, via Eurail or alternatively via Belgian railways (pass cover number needed, request one here).