That's not sufficient for many home designs, though; you may need to keep the interior temperature into the 40s or 50s to keep the pipes in the wall above freezing, depending on the outside temperature and your house design. One reason the default "away" temperature for many thermostats is actually pretty high, like 55 or 60.
True, and just "learning" your regular day-to-day habits would not necessarily inform the system about an absolute lower bound on temperature.
I think the thermostat control problem is harder than I might have given it credit for. Any time you have to learn in a nonstationary environment (http://mathworld.wolfram.com/NonstationaryTimeSeries.html), nasty surprises can happen. The future may not look like the past.