if you narrow it down, blackovis makes returning stuff easy as long as you use some common sense not getting them dirty in the process of trying them. all you need is a steep slope or stairs in a pinch to see if your toes touch walking downhill or heel rub walking uphill... a slope you can sidehill is helpful too, make sure they are sure footed on a steep sidehill, and if you have any painful pressure points sidehilling that you cannot resolve by different lacing techniques.... if you have a pair of boots that's almost there, youtube some different lacing techniques and see if that helps, it can make a big difference in boot comfort.
the lighter and more flexible, the easier it will be to find comfortable boots, but that's not the best option for many folks, some need a stiffer boot or one with more support, and boots like that are more temperamental with fit, and sizing is a lot more critical.... you can't get away with much with stiffer boots, if they are too big or too small, you WILL suffer on the mountain.
you hear a lot about break in, in my experience, if a boot sucks on your feet on day 1, don't expect breaking them in to make them comfortable, i have kept lots of boots i didn't like thinking they would break in... they do, but that doesn't make them comfortable.
insoles/orthotics can greatly change the feel of a boot, if the discomfort is because the arch is wrong for you, that can usually be remedied with an aftermarket insole like superfeet, probably much better with a custom orthotic, but i have not had them yet, so i can't give any testimonials there. the superfeet copper fit my foot profile well, so i can put them in any boot that fits me well and have a comfortable boot