Ah, so you are starting the day in Luzern. That explains why you don't want to backtrack there on your way to Lake Como.
Is going up the Jungfraujoch a lifelong dream of your mom's, or just something she heard was fun and a nice view? If the latter, you may want to reconsider. The time and cost are significant--- 4 1/2 hours and 193.80 CHF RT from Interlaken if you don't have a pass. Add 2 hours and 30 CHF to travel from Luzern. That is over $200 for each of you, and 6.5 hours spent on the train (starting at Luzern and ending at Interlaken). Add 2 hours on top and I can see why you can't start for lake Como until late afternoon.
As an alternative, consider one of the excellent mountaintop experiences closer to Luzern (and much less expensive).
There is Mt. Pilatus, a rocky summit right above Luzern, with wonderful views over the lake. You can go up and down by cablecar from Kriens, a suburb of Luzern, or do the "Golden Round Trip" by boat on the lake and cogwheel train to the top, returning by the cablecar.
http://www.pilatus.ch/default-n7-sE.html
Tor a glacier and snowy peak, try Mt. Titlis, above Engelberg. This is a 1-hour train ride from Luzern. There is a grotto carved in the ice, and views galore. Description from a tourism site:
http://www.myswitzerland.com/en/offer.cfm?category=Market_Specials&subcat=IN&id=91506
www.titlis.ch
Or you could go up Mt. Rigi, which is much less rugged and not covered with snow, but offers some nice walking, and is right on your way. You coul dactually go up and over Rigi on your way to lake como: lake boat to Weggis, cablecar up to rigi Kaltbad, cogwheel train to th etip at Rigi kulm, and cogwheel train down the other side to Arth-Goldau, where you meet your train heading to Lake Como. You could take a nice walk at Rigi Kaltbad or Rigi Kulm, either one with great views over the lake and surrounding mountains, but you'd have to figure out what to do with your luggage in the meantime.