The pass should be a good deal but always compare advance tickets (with fixed departure times) vs Eurail + reservations whenever required. Some timetables are not yet available for July.
With Eurail you'll need to add seat reservations on Paris - Brussels (27€) and couchettes/sleepers on the night train. See https://www.seat61.com/interrail-and-eurail-reservations.htm
Note that an Amsterdam - Salzburg night train doesn't exist. There is Amsterdam-Innsbruck or Amsterdam-Vienna (via Passau). I'd suggest Innsbruck -> arrival around 9:30am (+ usual delay) so you wake up at around 7:30. From April they should use brand new carriages.
Then it's 1h30 with great views to Salzburg (possible to do an even more scenic detour via Zell am See). When the night train arrives at 7:30, staff usually wakes you up at 5:30....... :/
Travelling as a couple you might want a double compartment. Expect to pay around 300€ (so 150€ pp) on top of your pass. Book as soon as tickets are released. Sadly Nightjet is quite expensive these days. A good alternative would be mini cabins if the new carriages are operating out of Amsterdam by then.
You don't need a 15 days pass. The pass isn't valid on urban public transportation like trams, subways or buses. Some suburban trains are included though. On your itinerary I count max 7 days with train travel (days 2, 3, 5, 6, 7, 9).
Last thing: with travelling in the evening, you have to store your luggage in some place. Not always easy/cheap. Most people prefer travelling in the mornings/around midday going straight from check-out to check-in. You're also not as relaxed thinking throughout the day about the train journey.
