"If catching an 11:00 train and relying on the bus to get you back to the station, I think the timing might be a bit tight."
I agree. Remember that the bus between the Falls and the Lauterbrunnen train station only runs twice an hour, and you have to walk 5-10 minutes from the exit of the Falls to the bus stop. All that time (waiting for an elevator, taking the elevator, walking down through the Falls to see them, walking from the Falls exit to the bus stop, waiting for the bus, the bus ride itself) adds up. Even if you make it, you'll probably spend much of your Falls visit looking at your watch and worrying about your train, rather than enjoying the Falls.
I'd just walk around the area near your hotel and enjoy your last views - that's what I've done when on the mornings of my two departures from the Berner Oberland, and I haven't regretted it!