I think that BenD must be right in the above post that this is a common mistake in German, apparently even among native speakers (I assume).
In the one Google example I gave, it sppears to me that the author of the article made an error. What he said does not fit into your explanation that in these uses of "ersetzen" with "mit", the "mit" means "mittels".
That article has the following headline:"Finne ersetzt verlorenen Finger mit USB-Stick als Prothese
If what you said were applicable to this example, and "mit" meant "mittels", then in this article, the USB-Stick is the means he used to replace his finger. But what then did he use at the replacement. It seems here that the USB-stick is the replacement, not the means of carrying out the replacement.
Here's another example from: http://article.wn.com/view/2010/03/12/frau_er...
The article is entitled: "Frau ersetzt Mann mit Doppelgaenger"
Here, it seems to me that the Doppelgaenger is the replacement, not the means of carrying out the replacement.
Are these really grammatical mistakes, or is there something different about these examples that makes "mit" correct in these contexts. I would have thought that "durch" was correct here.