I had a play in a game last season where the QB threw the ball to an offensive lineman who was 5 yards downfield. So we had ineligible receiver downfield and illegal touching. It seemed like an obvious enforcement as the ineligible receiver penalty is 5 yards from the previous spot while the illegal touching penalty is the same but also includes a loss of down.
But later I wondered if we might also have had a foul for an illegal forward pass. The QB was being pressured when he made the throw. I don't recall whether there were any eligible receivers in the area, so I think that would have been the deciding factor for that ruling. That would have also been a loss of down, but the next play would have been from the spot of the pass. Do I have that right?
Yes, the legality of the pass makes all the difference. Illegal touching and ineligible downfield fouls can only occur during LEGAL forward passes, by rule. If it is any form of an illegal forward pass, including intentional grounding, then there can be no illegal touching or ineligible downfield fouls. So, the first thing the crew must do is determine if the pass was legal. If it was, then, yes, you could have ITP and/or IDP. As you say, you'd decline the IDP, and accept the ITP, since it includes LOD.
But, the pass could easily be ING, so you must go through the process of determining if the pass was ING or not. That's a whole other story, that we'll defer to another time. But, if it was legal, then you may have ITP and/or IDP. If the pass was not legal, then you'd only have the ING or IFP.
Almost any time you have an ING, that will probably be the penalty of choice, because, being LOD at the spot of the pass, it will almost always result in a greater yardage loss than even a holding penalty, plus it includes LOD. Even other illegal forward passes will likely trump other penalties. EX: Second forward pass, or a pass thrown beyond the NZ - that's 5 yards from the spot of the pass, plus LOD.
So, yeah. You gotta know the status of the pass. Totally OK to wave off the ITP or IDP, if the pass was illegal. (Heck, you might not even signal them, at all - just go with the ING or IFP).