Help for first-timer

You should book accommodation in advance (especially in summer) but you can always choose refundable options. Paris is hosting the Olympics so expect very high prices, more crowds and extra security measures.

I’d compare regular advance tickets vs pass + reservations as they’ll cost a fair bit extra. Have a look at https://www.seat61.com/interrail-and-eurail-reservations.htm

Eurostar has a passholder quota (30€). Book well in advance, especially with the Olympics. The quota works by “buckets” : when tickets reach 170€ passholder reservations stop being sold. Evening trains are often cheaper and thus have more availability.

San Sebastian makes more sense via Bordeaux yeah and you avoid the expensive/too popular Barcelona TGV.

Questions welcome.

1. Eurostar is not included in the Interrail pass. You'll need to purchase a separate ticket for Eurostar travel between London and Paris.
2. The online map provided by Interrail is generally accurate, but it's always a good idea to double-check specific routes and stops. You can indeed stop at Bilbao or San Sebastian before Madrid, but it's essential to verify if these routes are covered by your pass in advance.
3. It's recommended to plan your accommodation in advance, especially during peak travel seasons like August. Booking early ensures you have a wider selection of options and may secure better rates.
4. Remember to validate your pass before boarding trains and familiarize yourself with any reservation requirements for specific routes or trains. Additionally, consider travel insurance for peace of mind during your journey. Enjoy your trip!

This is wrong and you've already been corrected multiple times.

You do need a seat reservation on Eurostar services, like for most high-speed trains but Eurail/Interrail is fully valid and you only pay the passholder fare.