Fabius -

I am a DNI site addict and I have enjoyed all your articles posted there. This one is no exception. It is good to have you here.

I think a mercenary/militia mix could be ideal.

As for the deniability of a mercenary force - I'm not so sure. To hire mercenaries for any purpose, a president would have to seek congressional funding and he would have to have an estimate of the cost along with the thinking behind the estimate. It would be a built-in check/balance in that the president would find it difficult to hide his true purpose when he has to open the project up for discussion with the representatives of the people. Not impossible, just more difficult. I would contend that congress voting moneys for a president to wage war as he sees fit and giving him a blank cheque to do it is something that has already been done on several occasions since WWII anyhow - a mercenary service would not make that more likely.
In the current structure, the NSA or CIA or who knows what other orgs can, in effect, act as such mercenaries at the whim of the president....I don't see how this would be any different. If anything, it would be harder for him to wage war without more congressional oversight.