Hello Nocoalee35
From Interlaken to Luzern, seat reservations are not mandatory and, except on peak periods, not necessary. These trains have usually only panoramic cars and everyone seats in these cars, tourists and locals.
The tickets will be checked in the train, not the seat reservations. You can see on displays if seats are free or reserved. You can also choose other free seats if you find better ones.
Point-to-point tickets can be use in every train during the day of validity for one trip between the two stations.
Kind regards
Danhiel