Then what is "We have an IFR inbound, hold for release?" If you're taking off from an airport in class G, ATC can still make you hold and not give you a clearance. So they do have control over uncontrolled fields?
So based on this IFR in class G stuff.... does that mean that any instrument rated pilot caught breaking VFR wx minimums in class G airspace can always get away with saying he was IFR without a flight plan? Basically any instrument rated pilot can fly IFR in class G at any time?
What is IFR anyway? I don't think it's clearly defined anywhere. Part 91 basically just talks about fuel minimums, approach minimums, takeoff minimums, cruising altitudes/headings, and communications. It doesn't ever define instrument flight rules or what "IFR conditions" are. First we need a clear definition of what instrument flight rules are before we can determine if they can be flown in uncontrolled airspace.
By not releasing you, ATC is preventing you from entering controlled (class E or greater) airspace under IFR. If you can maintain VFR, you could still take off and request your clearance airborne, but ATC will not issue the clearance until they can provide positive separation. In most parts of the US, If you could not maintain VFR, you wouldn't be able to takeoff because you would not be able maintain IFR terrain and obstacle clearance while staying out of the overlying controlled airspace, which starts at 700' AGL over airports with published IAPs.
Regarding the second and third questions; I like to think of IFR as any time VFR rules are not being maintained, and/or when on an IFR clearance. If operating IFR, you must follow all IFRules, and all of the rules of the airspace you are operating in.
With rare exception, IFR in class G can only legally occur without a clearance in sparse areas of the US where G goes all the way up to 14,500'.
Clear as mud?