Let me see if I can follow your discussion . . .
Case 1: A squad possessing a MG fires only its Inherent FP in Defensive First Fire and then doesn't SFF at all.
In the DFPh, the MG can fire with full ROF and full FP and full range and no Sustained Fire penalties--and once the MG's ROF ends, it can't fire any more.
The squad can fire its Inherent FP, but only against adjacent enemies with Area Fire.
If the MG and Inherent FP are used together in the DFPh, fire is limited to adjacent enemies, and the squad's FP is Area Fire, but I think the MG would retain full FP, full ROF, and no Sustained Fire penalties and could fire elsewhere afterwards if it retains its ROF.
Case 2: A squad possessing one MG fires the MG and its Inherent FP during Defensive First Fire and then doesn't SFF at all.
In the DFPh, the MG is limited to Area Fire, adjacent enemies only, no ROF, and Sustained Fire penalties.
The squad's Inherent FP is limited to adjacent enemies with Area Fire.
If both the Inherent FP and First Fired MG fire together, fire is limited to adjacent enemies with Area Fire, no ROF, and with Sustained Fire penalties.
Case 3: A squad possessing two MGs fires one MG and its Inherent FP during Defensive First Fire and then doesn't SFF at all.
In the DFPh, if the squad fires the unmarked MG, it can fire with full ROF and full FP and full range without Sustained Fire penalties.
The squad's Inherent FP is limited to adjacent enemies with Area Fire.
If both the Inherent FP and unmarked MG fire together, fire is limited to adjacent enemies, and the squad's FP is Area Fire, but the unmarked MG would retain full FP, full ROF, and no Sustained Fire penalties (and could fire elsewhere too, if it keeps ROF).
That's how I read the rules, but I can be wrong somewhere.