The short answer to your question is YES. The extra acid in the ingredients will hamper the second act of the double acting baking powder. The acids are timed/staged for reaction not the baking soda.
The Magic Baking Powder (happens to be in our kitchen, too) is mostly a single acting formula since monocalcium-phosphate is a low temperature acid (with apparently some double acting properties due to generation intermediate step of dicalcium phosphate; per your link). High temperature acid for second acts typically include sodium aluminium sulfate, sodium aluminum phosphate and sodium acid pyrophosphate.
You can try to counteract that by adding a bit of baking soda, but you run the risk of altering the taste and not having it all neuralized.
If you really want to get pedantic, use a pH meter to measure the acidity of your dough. I suspect tasting the dough might give an indication as well. (bitter alkaline, sour acidic)
Another test might be to mix your acidic ingredients in a bowl with some water and start adding measured baking soda until you see no more reaction (bubbles) and use that as a your basic of neutralizing your dough.
All said and done, I agree with SAJ14SAJ that you'll be just fine going with the existing recipe. There should be enough baking soda left to get something out of your double act. I also heard it from a world-class baker that most recipes can be done with only baking soda, let alone baking powder or double acting ones.
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.)
Best Answer
Baking powder is like a fast-acting yeast; it is used to infuse air into baking mixtures by way of carbon dioxide bubbles, created by a base reacting to an acid. Baking power is made of three different parts:
All three need to be dry powders that can be mixed together, common ingredients are cream of tartar (acid), baking soda (base) and corn starch (filler). The role of the acid and the base are to combine together to produce carbon dioxide bubbles when reacting with water or other liquids. The filler helps keep the baking powder dry, so that it remains free-flowing and so that the base and acid don't get moist and interact in the container.
Resources: