The Brit seems to be the only one that actually agrees with the Purpose of the Well, that's not hte same thing.  The others seem to LOVE to communicate because without exception they seem to think they might be able to use it to get free.  Which is my point, Any contact at all is potentially a Security Risk, so if you are designing a 5th dimensional Supermax, why risk it?

What the inmates of a Prison think of it's design doesnt really matter...Im sure they'd all vote for short walls made of cheese

On the flip side, communication does not necessarily equal magic or influence.  With Harry running those tunnels for months, anything could have influenced him.  I think instead the crystals allow word/thought communication to pass through to the warden when he (or previous she's) are in close proximity, but nothing even close to the line of magic.  This is why the island still stands instead of having had a massive prison break eons ago from prisoner x using their abilities to shatter prisoner y's crystal.  Now manipulation with truth (a la Changes and Mab) is another story.

most of the discussion so far has centered on the prisoner being Merlin.  Arthur may definitely fit.  I'm just trying to rectify his significance to the future of the dresden files.

I don't think it could be done in changes.  Not only not by Harry, but not by anyone.  Per Bob in CD, the mortality of immortals on Halloween is what would have allowed it to work.