On the route you outline you'll only need compulsory reservations between Zürich and Paris and between Paris and London.

• For Zürich-Paris, the reservation fee on the direct TGV is considerable (€29 in 2nd class, €39 in 1st, both per person). Alternatively, consider booking seats from Mulhouse or Strasbourg to Paris (those should be available from €10), and swiching trains in both Basel and either Strasbourg/Mulhouse, using a regional train across the border. That isn't very much slower, but does involve those 2 train swiches.
• For Paris-London, book the Eurostar as soon as you know your dates. There are limited seats available for passholders, and no realistic alternatives. However, reservations are only limitedly flexible. These can be booked from: https://www.b-europe.com/EN/Booking/Pass#TravelWish

All other routes you list don't require reservations, but some do have optional ones. 

Is there a risk if we buy the Eurail pass in the next couple of weeks that we could miss out on our preferred day and  time of travel ie 11 am on 2nd Jan?. How do we know if 4 seats are available before we buy the Eurail pass? 

Regards 

Richard

For Eurostar check here : https://www.b-europe.com/EN/Booking/Pass#TravelWish

For all other trains check whether tickets are still available through the railway company. Trains cannot sell out in the UK, Austria, Switzerland,...

Eurostar should be booked soon but all other trains can wait. It's likely that you can't book them right now anyway with the big 2024 timetable change.

Thanks so much to the Eurail community for your help so far. I am sure I will reach out again and from Australia first time travelling to Europe during winter and at Christmas with family is going to be an adventure. 
Richard

I assume b-rail is the booking agent for the euro star? It would appear the cost for 4 of us is $120 Euro in addition to the Eurail pass at our preferred time . Does that sound correct? And same booking fee for adult or youth? 
 

B-Europe is the reseller of the Belgian Railways. They add 4€ fee per order compared to 2€ per person per train on eurail.com. It is often cheaper to book elsewhere than Eurail due to extra fees + more expensive optional reservations. More info :

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

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

For example : Salzburg - Zurich (optional but recommende reservation) costs 3€ on ÖBB but 8€ on Eurail. Seat reservations in the UK are optional and free of charge.

Eurostar is expensive yes (but there's no other rail alternative...) : 30€ per person from France/Belgium, 35€ from the Netherlands.

Seat reservations generally cost the same whether for an adult or a child.

For the UK trains:

London-Brighton can’t be reserved, just turn up. Avoid rush hour and you shouldn’t have a problem getting a seat.

London-York-Edinburgh-London - assuming you are travelling on LNER (the main operator on that line) you can make seat reservations free of charge on https://www.lner.co.uk/travel-information/make-a-reservation/. Reservations aren’t really necessary, but since you’re travelling around the post-Christmas period, it can’t hurt to have a reserved seat. You can still jump on an earlier or later train if you want.

(You can make reservations for any reservable UK train at gwr.com, but you may as well use LNER if you’re travelling with them.)