I read your article thoroughly and it seems a good idea.
Actually I have started thinking that it will be more or less centralised currency whatever you gonna name it. Because you are controlling it with the use of it within your marketplace. Its like google play cards preloaded with the USD. That mean each card having specific value. In your case well you are having your own coins with equivalent USD at the launch or after the launch. Im not sure if you are completely getting it but your statement here says something similar:
People can buy apps on our marketplace and pay for them with our token. Tokens are purchased from a public exchange from investors, and hence this creates real customer demand for the token and expands the number of people holding the token - theoretically resulting in an appreciation in price.
So speaking about the supply you will actually provide the tokens as and when required by the user and you also not mentioned how you will circulate them? Like with exchanger or by the marketplace checkout itself? Thats going to be centralised I guess.