CAN you depart "IFR" from a non-ATC airfield, anyway? You can't be in receipt of IFR departure instructions, for a start.
Yes.
It is not mandatory to be under a clearance to operate IFR outside controlled airspace. If the departure is made, remaining clear of controlled airspace, then the fact that the A/G Operator cannot give you a clearance is neither here nor there.
The Rules to be obeyed under IFR are set out in the Rules of the Air.
2D