Haley, my first impression is that you are trying to pack a lot in just to make the most of your time. Often the best way to make the most of limited time is to not try to cover so much and remember the adage that less is more. Having said that, what you are planning is possible, but you will probably not have much time to savor each destination. Your October 19 plans look particularly crowded given the in-transit time you'll have to include in your schedule. (E.g., train from Interlaken to Lauterbrunnen, cog train to Wengen, cable car to Mannlichen, cog train from Kleine Scheidegg to Wengen, cog train to Lauterbrunnen, bus to Trummelbach Falls and back to Lauterbrunnen, train from Lauterbrunnen back to Interlaken.)
Average October temperatures in Interlaken are in the upper-50s to mid-60s for highs--cooler later in the month. Lows are in the 40s. It will definitely be colder up higher--Mannlichen and Kleine Scheidegg--and possibly downright frigid on Jungfraujoch, expecially compared to your Houston standards. Weather can be "ify" in the region in later October--snow in the mountains, for example, rainy in the valley. Given the vagaries of weather that time of year, you'd need to be flexible with what you do. The Mannlichen-Kleine Scheidegg hike may be out of the question as may Jungfraujoch--it will depend on the weather. Remember, too, that daylight will be shorter.
If, because of the weather, you need to bail out of the Interlaken/Lauterbrunnen area, consider going on to Lucerne and spending more time there. There is lots to explore in Lucerne, and the half-day you'd have with your present schedule is not much. Interlaken is not a good place for a "rainy day" unless you like overpriced restaurants and souvenir shops. If weather (i.e., snow) does not allow hiking the Mannlichen-Kleine Scheidegg trail but you still want to hike in the area, consider the walk from Lauterbrunnin to Stechelberg with a stop at Trummelbach on the way. It's beautiful.
