So it is often cheaper to book your reservations elsewhere than on interrail.eu.
More info about seat reservations in general : https://www.seat61.com/interrail-and-eurail-reservations.htm
Eurostar/Thalys to Paris : 32€ through https://www.b-europe.com/EN/Booking/Pass#TravelWish (4€ fee per order). You're lucky that there are still seats left, usually this route sells out days or weeks in advance (passholder quota too) !
TGVs within France : 10€ limited fare, then 20€. Use https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish
The RER from Gare du Nord to Gare Montparnasse is not included. You should be able to buy tickets onboard the Eurostar so you don't have to queue in Paris !
It would be cheaper but a bit longer to go via Strasbourg. Just pointing that out as an alternative. :)