<<"If a choice is given to you to cut back on one of the bases to have more
time in Interlaken and Lucerne then which one you will pickup?">>
I'd add in the Jungfrau region, as noted below.
And, that extra day in Luzern that you note will allow some local sightseeing, as noted below.
Your itinerary has allowed suitable time for travel between St. Moritz and the Jungfrau region, and generous time for travel between the Jungfrau region and Luzern.
The timetable and map will help you to understand that:
From St. Moritz to Interlaken Ost - 5 1/2 to 6 hours,
From Interlaken Ost to Luzern - 1 hr 51 minutes
IO to Grindelwald adds about a half an hour.
After you have obtained a list of trains/connections by entering locations, date and time of day in the timetable, you can select a sigle journey by clicking on the "+" to the left.
The details of the journey and stations for changes will appear. In the footnotes, you have the option to see a map.
For an overall map of Switzerland, this one is particularly useful:
By using the menus and zooming, you can get much detailed information about the train stations, etc.
I'd add one more day in the Jungfrau region, after day 14, to allow for a better chance for good weather up high. The weather changes rapidly in the high alps, so a cloudy morning might have a sunny afternoon, or vice versa. Or, the next day may be better. There are lots of ways to keep track of near term weather once you are in the region.
I'd fit in some time to walk around Luzern on one of your days, or, perhaps, the afternoon of the day that you come from the Jungfrau region. There is a lot to see and do, good weather or bad.
Personal preference enter in my last comment. I'd take the time around Luzern instead of the Rhine falls. However, many other enjoy the falls more than I do. In the USA, we have Niagara Falls, so the Rhine Falls are not so special for me.
Luzern and Zürich are less than one hour apart by frequent trains. The falls are about an hour from Zürich:
So, you can be flexible with your times on the last day or so.
Hope this helps,