Several points I would make about boot fit, I also have flat feet.
1. I find that riding any less than 1x a week and my feet muscles just cannot keep up. When I ride 2 days a week or more, my feet NEVER hurt; but when I go once every 10 days, or at the beginning of the season, the small muscles in my feet really give me alot of pain.
2. Riding more also allows you to ride more correctly, and use the connection between the top of your boot and lower leg to turn, allowing your feet to just relax in their snug little cradles. (If you are trying to control the board with your feet, not much point in using highbacks)
3. Boots and bindings should be snug, not tight, overtightened bindings cause foot pain.
4 & 5. big toe shop fit heat molding nonsense story: heat molding your boots in the shop help you to feel how the boot will fit as it breaks in (not packs out). Don't be oversold on the concept, when the boots are completely packed out its time for a new pair, you are buying them for how they support and love you brand new, right now. The entire time that you wear them they are gonna be 100 degrees farenheit constantly heat molding. Back to the big toe issue. Stand in your boots with your feet shoulderwidth apart and bend your knees so that you squat about 4-6". You will notice that as you bend your knees, your toes will pull back from the front of the boot. In a perfect fitting boot (unless you are shopping it as a hiker who plans to put miles in them) your toes will actually slightly push
against the front of the toe of the boot in the standing position, and as you bend your knees and they pull back, they will never push against the front when you ride, but still fill the toebox comfortably and perfectly. This is counter-intuitive to any sort of shoe or boot buying in normal life, however it is true.
6. Different brands and even models within brands fit differently, perhaps that model of k2 in those sizes is not a great fit. A bootfitter can also help.
7. I rode 50 days in pain last year in a pair of k2 T1 DB's, a top of the line boot that was my size. This year I got 2 pairs of 32's, the Lashed and TM-Two (these are a little stiffer than the lashed with a considerably thicker liner) and they both fit me much better with their liners swapped. The Tm-twos are a bit too stiff and tight for my fat foot, so the thinner liner of the Lashed allows the perfect amount of room. On the flip, the Lashed with their own liners are a little swimmy and loosey goosey, and with the much burlier liner from the TM-two's I get a really nice fit. Hence I enjoy benefits of frankenbooting.
8. Keep your toenails VERY trim, always, seriously.