The Saver Day Pass covers public transit. The boats on the lake are considered public transit and are covered, but the Harder Kulm funicular is not, and neither is the Firstbahn. Rule of thumb: If it goes to places where people live it is public transit. If it goes up a mountain where there is only hiking trails and maybe a restaurant, it is tourist transport, and not covered by a public transit pass like the Saver Day Pass.
So on 5/7 you could just buy a roundtrip ticket Interlaken - First at the station in Interlaken, and that would cover your train to Grindelwald and the cable car ride.