On the question of venerating icons etc., those who defend such practices would of course say that they're not worshipping them, they are worshipping God through them. So it's not idolatry, because only God is being worshipped. Of course you might not accept such a distinction.
On the question of the development of doctrine or practice, that's rather more interesting, I think. It's arguable to what extent there has been such development in Christianity. The seventeenth-century Catholic theologian Jacques-Bénigne Bossuet agreed with your criterion that all such innovation would indeed be "contamination" and wrong. But he thought that it had never happened in Catholicism, and that Catholics had always believed and done exactly the same things; and the only innovators were heretics (e.g. Protestants). That's not a very plausible view today. The Catholic Church developed instead a theory distinguishing between legitimate development and illegitimate development; the key figure in this was the nineteenth-century theologian John Henry Newman. Newman thought that development is OK provided it's true to the roots - so, for example, the first Christians didn't venerate Mary and later Christians did, but this is OK because the reasons for venerating Mary can be found in the earliest Christians. It wasn't an innovation, simply a logical working-out of what Christians had always believed. At least, so the theory goes.
