<<"as our flight from Geneva to Zurich is non-refundable.">>
If you did not want to see the Lavaux region, I'd say throw it away.
Even if you do...do the Lavaux before you go to Interlaken and never go back. Throw it away.
It is clear that you did not understand the logic.
I am asking you to set prioriteis. Is making the flight a priority, or seeing more of Switzerland?
To clarify...your journey in Switzerland fits the definition of "Project" as industrial planners use the word.
A project is an activity with a start, a finish, a budget, and a result.
Within the constraints set for the project, you may wish to trade off one or another activity for something else. If so, there must be give and take. For example, to make the trips faster, you could rent a helicopter. But, your trade off would be to add to the the expense.
The famous rule is "Good, fast, cheap....pick any two." Good refers to the events or quality of results. What it means is, If you emphasize one of the items, one of the other 2 has to give way. In your case, the total available time cannot be changed, so you would be trading off within that constraint.
In your case, you are degrading the quality of your trip by trying to save money ...that is, by not wasting the cost of the GVA-ZRH airfare.
In fact, if you throw away the air ticket and take a train from Interlaken to Zürich Airport, you have no extra train fare. So, it costs you no more.
So, in one sense, you trade of the cost of the discarded air ticket and taking the flight, for an improvement in the quality of your trip. No ride back to Geneva, no need to get to the airport early, and more time where you want to spend it, in the Jungfrau region.
I have no idea what the airfare from GVA to Zürich is, but if you spend time on a trip that you do not need to make, you degrade the overall quality of your holiday.
Your choice. I 'm just telling you what I would do , personally.
(Also, I'd check to make sure that skipping on leg of an air trip did not force a cancellation of the last leg....it could happen.)
As for your other ideas, use the timetable to see how they fit. With another day in the Jungfrau region, you would have a better chance to try them.