Hi OhioKid, and welcome to MySwissAlps!
>> Primarily want to explore ... Schilthorn ( for Jungfrau massif)
I'm not quite sure what you mean by this. You can see the Jungfrau massif in the distance from the Schilthorn, but if you want to go to the Jungfraujoch, the train that takes you there is on the other side of the Lauterbrunnen Valley from Mürren and the Schilthorn.
>> and the train ride between interlaken and lucerne. Additional interest Zurich and Geneva Lake but will compromise any of those if spending more time for Alps.
Given the time you have available, I would skip Zürich and Lake Geneva, as Luzern and the Jungfrau Region have so much more scenic beauty in the immediate area compared with Zürich and Lake Geneva, and you can easily include a lake cruise in either Luzern or Interlaken.
My suggestion would be to split your time between Luzern and the Jungfrau Region (Grindelwald, Lauterbrunnen, Wengen, Mürren). In terms of travel times in from Venice and out to Paris, it doesn't make all that much difference whether you start in Luzern or the Jungfrau Region.
>> Please can you suggest an itinerary. Is there any day tour offered by any tour operator locally?
Excellent public transport is available to all the main attractions, and many areas are car-free, so tour buses cannot go there. In my opinion there is no advantage in taking a guided tour, and I imagine it would cost you more.
There is plenty of information (including links to local attractions with directions for getting there) on the following pages:
To plan your trips to local attractions and between your overnight bases, you can use the Swiss Timetable: