Freeze chilli powder

Yes, they can be, but you'll want to prick each one with a sharp knife once or twice to prevent them from bursting. Once that is done, put them into a small freezer-safe storage bag, remove as much air as possible from the bag, and seal it. They keep reasonably well for about six months without tasting "burnt," but they tend to be a tad mushy upon defrosting.

I always thought the mushiness factor had to do with length of freezing time, but since you asked, I did a bit of research. Apparently, what makes chilies turn to mush actually has to do with how quickly they initially freeze:

The undesirable mushy end product that comes from freezing fruits and vegetables isn't from the act of freezing itself, but from how you freeze. Your ordinary freezer, or even deep freezer, freezes food slowly. Chilies have water in their membranes, and as they slowly freeze, it forms jagged ice crystals that pierce and puncture the membrane, so that when you defrost it, you're left with a leaky and "tenderized" result. The solution to this problem is easy … faster freezing. If the water doesn't have time to form jagged ice, you won't have a mushy membrane.

Happy Living (from which I pulled the above quotation) has an article about how to safely use dry ice to freeze chilies quickly. (I'll probably keep going the way I always have and accept the mushiness, but if you want to freeze a lot of chilies for a variety of applications, the dry-ice method looks promising.)

From experience I'd say those 6 months don't stack. You could have 6 months in the jar, or 6 months in the freezer - very soon after defrosting the flavour would die out. If you have to freeze I would recommend using it straight from the freezer, if you can avoid it clumping.

The overall potency is degrading from the moment it's picked! But realistically from the moment of grinding, as the oils in the ingredient are slowly seeping and evaporating out of the grain.

Why not make smaller batches more regularly? That's what I do for garam masala, mixing a new batch every 3 months or so.

I would always recommend oven drying spices / peppers very low for a long time - say 2 - 3 hours at 100C (212F). Wet ingredients could be dried in the same way, else added at the time you are going to cook with them.