The name space is not a problem. You can add as many name spaces to an XML document as you like. That’s the “X” in “XML” (“extensible”).
I must admit I have no idea why the iTunes directory rejects the feed. iTunes the app is happy with the feed as is the Castfeedvalidator and like probably every podcast app on the planet.
Don’t trust the old Feed Validator. We have been telling the team not to put up name spaces as errors (they are not an error), but they are totally unwilling to change anything. It would be better to delete the web site as it is not helping.