Hello JayLB
Welcome to the SBB Community.
Swiss train tickets never come with seat reservations. We locals use our trains like others use buses in their city: by hopping on / off as we like; the obligation for seat reservations would mean a great hindrance to us.
Please read our guide on this subject:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
Here is the link to make optional seat reservations on the LIX, Luzern-Interlaken-Express:
https://store.luzern.com/luzern_en/specials/daytrips/zentralbahn-int-lu
A Saver Day Pass allows you to use the entire public transit network at will for the whole day. You just walk in or hop on / hop off as you like.
Apart from Supersaver tickets and at least for domestic travel, the selection of a particular connection during the ticket purchase process does not mean, you have to take that train as indicated on the schedule. The ticketing system needs your selection of a particular connection in order to calculate the available ticket offers; for longer distances the Saver Day Pass is one of them when the ticket is purchased online.
Kind regards,
Roland