Neither form, matter, or intention of the NOM are necessarily doubtful.
The minister may or may not be, depending.
Consequently, the NOM is not in every instance doubtful (but remains evil despite possiblity of confecting a valid sacrament).
I find the honesty of your rejection of ABL quite refreshing, despite my opposition to it.
That they are questionable is enough to set them as off limits. Sean, that is the very point, if one cannot be sure of the validity of any given priest, then the validity of the sacrament must be held a doubtful, because the Church always requires us to take the safer course in these matters.
Telling someone that they can attend an un-Catholic service with a possibly or likely doubtful priest is bad advice and goes against the doctrine and long held practice of the Church.
What can I say? It is an the truth. It is irrelevant what the subjective circuмstances are, only that the objective danger is definitely present. That is what the Church is concerned about.
The laity is neither theologically nor experientially skilled enough to make such determinations on their own, and should certainly not place their souls in peril based upon the opinions of clerics,but rather upon the sound framework of the Magisterium.