Hi Slothrabbit -
Near the Jungfrau, you have not included a trip up to the Jungfraujoch ( I know...discussed earlier, just mentioning it, may be altitude problem) or Schilthorn (altitude?). It is wise to have some slack for bad weather, and so you might not want top cut back your time there. As you note - Bern is good for a cloudy day, and I note another alternative below - the Rhone Valley.
This map can give you altitudes:map.geo.admin.ch/?topic=ech&lang=en&bgLa yer=ch.swisstopo.pixel karte-farbe&layers=ch.bav.ha ltestellen-oev,ch.swisstopo.swiss tlm3d-wanderwege&layers_visi bility=false,true&X=15 8353.49&Y=635315.09&zo om=5
It also shows hiking trails.
The walk from Wengen <=> Kleine Scheidegg is not my favorite. Check this thread where I have posted some maps and discussion with my July 26th post. 8oo meters elevation change.
Day 7 could be a good day to go up to Schilthorn from Mürren. there is not a lot to explore in Mürren. If you want to see a bunch of tourists with Rick Steves' book in their hands, youi coud make the short trip to Gimelwald, just below Mürren.
One way to deal with the situation instead of definitely adding a day at Luzern might be to leave for Vevey a day early as a function of all you have seen, or bad weather. There is plenty to do near there, or, move one day from there to Luzern. In October, most places have rooms available on short notice.
The Rhone Valley -
Here is another possibility, easily done from Lauterbrunnen. It takes you to the Rhone Valley, which is usually sunny, and warmer and much drier that the north side of the Alps. Take a train via Spiez (change) and on to Hohtenn. Walk the Südrampe trail (technically, the BLS Südrampe Trail) to Ausserberg. Take the train back to Lauterbrunnen.
Here is why:
If you go, taking the train to Hohtenn from Spiez will automatically take you up the old, scenic route via Kandersteg, then through the short tunnel to the Rhone Valley. The train you pick up at Spiez is called The Lötschberger and runs once per hour (starts in Bern) . When you are looking at the timetable, you can enter Kandersteg in the "via" box as another way to force the choices to the upper route. That would be the appropriate action if you wished to go directly to Visp or Brig by the scenic route.
Otherwise, the timetable defaults to the fastest route through the new "basis" tunnel from Spiez to Visp. The views of the inside of the tunnel pass quickly...it is a fast train...and it is good thing, because they are not very scenic. It could save time on the way back, perhaps.
When you have finished your walk at Ausserberg, you can ride back toward Kandersteg ( the way you came from) or, take the train down to Brig, hop over to Visp ( many trains through Brig pass through Visp) and return to Spiez and Lauterbrunnen by the new tunnel. Could be faster, even though you start out in the wrong direction at Ausserberg. (Nice train ride down to Brig, though.)
Timing from Lauterbrunnen, on an arbitrary day in early October (Trains leave Lauterbrunnen at xx03):
0903 LB--> Hohtenn- Arrive Hohtenn 1059.
Use toilet at train station. Walk to Ausserberg.
Minimum time in a real rush - 3 hours. Don't rush; allow 4 hours or even morethan 4 hours if you take a long break at the beer garden. It is easy to get in a conversation there. Once you are at Ausserberg you can decide whether to go up or down ...whichever train comes next....although the ones down to Brig via Lalden only run 8 minutes after the ones that go direction Bern ( up the hill) and take about 10-15 minutes longer to LB, but it saves waiting an hour. There is also a bus down to Visp at xx32 and 59 with good connection for Spiez. (to see that in the timetable do not use via Lalden in the via box in the turntable ). There is a restaurant with a terrace at Ausserberg Station. Nice place for a beer after a hike, while waiting for a train. If you are going back toward Hohtenn and Bern (uphill) allow a couple of minutes to get to the tracks on the far side via the short tunnel at the station.
Depart Ausserberg for Spiez ( direction Bern) at xx48
Ausserberg at 1448 -> Lauterbrunnen at 1655
AB 1548 ( 5 hours for the walk) -> LB at 1755
Or slip your start in the morning by an hour if you are feeling lazy or want to come down from Wengen.
The nice thing about this hike is that hourly service from Ausserberg back to the Lauterbrunnen Valley runs at least as late as 2048 on the route direction Bern, so you don't have to rush.
It could be a good trip on a cloudy day.