Seat reservations using a Eurail Global Pass

I am not knowledgeable on all aspects of Pass use (in particular I don't know how to make reservations with them) but I'll comment on what I know.
Also, I'm not sure that the pass is the best deal, especially in first class, but since you bought it, I guess it's better to stick with it (even though I believe you can get refunds).

1. Italy long distance trains require reservations, but these reservations don't run out, so it's a very limited risk to make them once your plans are stable - even, once in Italy!

2. Venice to Varenna is via Milan, there isn't a way to reserve on Milan to Varenna (regional train)

3. Varenna to Interlaken via Montreux is not the best way, unless you sleep in Montreux and plan to visit that area. I don't love the so-called Golden Pass, I find the Gotthard Railway and the Luzern-Interlaken express to be more scenic. My recommended route from Varenna is to (I) get to Como San Giovanni by ferry or ferry and bus, then (II) train over the Old Gotthard Railway towards Luzern, then (III) on to Interlaken. (II) involves three trains: Como-Bellinzona-Erstfeld-Luzern but in Switzerland this is straightforward. Look at the SBB planner (Swiss railways) and look for the route from como to Interlaken via Goeschenen (to force the Old Gotthard route) and Luzern to see all options.

4. No reservation needed anywhere in Switzerland except for special scenic trains like Golden Pass or Bernina Express, and even there it's optional.

5. Reserve Thalys soon, it does run out.

6. Reservations recommended on Interlaken to Amsterdam, but not mandatory.

Balso, much thanks for your detailed reply. I've enjoyed many of your posts over time.

I've read many times here that the pass may not be the best value for everyone so I did a lot of research and it did pencil out. My wife and I always book 2nd class but the couple we are traveling with have never been to Europe (and didn't expect to return) and so we wanted them to choose whether to spend the extra $100pp for first class. BTW, we are trying to show them some of our favorite spots - thus the blitz at the end of the trip.
I'll be sure to take a look at the alternate route to the BO you proposed and all of your other suggestions. Thanks again for taking the time for your thoughtful responses.

Answers to all of these questions vary with the particular country and type of train.

You can reserve most trains through Rail Europe but not all are e-ticketed, some must be shipped to you. You can also buy in stations there as you go. The Thalys web site doesn't sell passholder reservations and Trenitalia used to but recently discontinued that due to some fraud issues. Things change. As already mentioned, Thalys is the one train on your list most likely to limit the number of passholders or to fill up. Thalys (and similarly priced, more-expensive-than-most) reservations are partly refundable before the departure of the train, with rules attached. Some cheaper, 4-5 euro seat reservations may not be refundable, especially if purchased through Rail Europe (for $11-12).

For the Golden Pass line, see http://www.goldenpass.ch/en/goldenpass/offer/view?id=82 to reserve the section between Montreux and Zweisimmen (the only part where it's really recommended).

German reservations are optional but can be made at bahn.com when you follow the link for Reservation only.

Laura, thanks very much for your thoughtful responses.

I think that answers most of my questions however more responses would be welcomed.

Standard reply to train questions: Find the basics, all of them, here:

www.seat61.com