Will the ingredients of "baking powder" store indefinitely?
For all practical purposes, yes. Just don't let moisture get near them. 

Both are a single chemical compound which does not react with air. The only thing you have to worry about is entropy, and it will not do anything bad for the next few decades. It doesn't matter if you mix and use the day after buying or years later, unlike the mix, which ages because the components do react over time. 

But you still will have suboptimal results when compared to commercial baking powder. In baking, you want a slow and even leavening action, which continues to happen after the cake has started baking. This is why today's baking powder uses multiple ingredients, some of which react at once, others at high temperatures. You cannot do this with your simple mix here. It will still work reasonably well (a century ago nobody had slow-release baking powder and they still made cakes), but you should try and see if the result is good enough for your standards. Also you will have to be more stringent with the baking directions (working very quick once the powder is moistened, not leaving a batch of batter stand around waiting for a free place in the oven, etc.)