Sucks to be Duracell
Sounds like you've got everything nailed. I'm familiar with most or all of the techniques you outlined and can't disagree. A major goal for my project was very low power, but that wasn't the only goal. It had to supply regulated voltage during the logging cycle and also provide the option of 3.3V or 5V. The 1MHz clock is not necessarily used for minimum power, it's to ensure the MCU is operating within spec when running on the battery only (which could be as low as 1.8V, which is the BOD setting I used). At any rate, it runs at 1MHz for relatively few cycles, just before sleeping. I had heard it said that 1MHz was the sweet spot for low power, but have not done the measurements myself so am not in a position to differ with your experience.
I'm really happy with the way the data logger works and the power consumption looks like it probably exceeded my expectations by a fair amount. The logging cycle can be as short as 15ms depending on sensors. Just received data from a couple loggers that were in Northern Michigan through the winter. The first was inside an insulated but unheated cabin where temperatures got down to 1°F. Its regulator was configured for 3.3V. It started with a less-than-fresh pair of AA cells, that measured 3003mV at the start, and 2987mV after 184 days. The second logger was in an open shed. It used 5V, and its battery started at 3255mV and ended at 2844mV after 195 days with temperatures as low as -22°F. It had more sensors and more complex sensors than the first unit; I don't have current measurements but I imagine its sensors drew 2-4 times more current than those of the first unit, and for longer intervals.
I can tell you get a kick out of the low power capabilities these MCUs have. So do I!