One final thought before I put all this to bed.
Until several years ago, a legal formation by the offense needed to have at least seven linemen (and therefore no more than four backs). The rule was written such that the number of linemen was specified -- they needed seven, at least. Since interior linemen are not eligible receivers by position, having seven on the line means that you had two ends + three backs that could receive a pass from the player who received the snap. Defenses could plan for and expected no more than five potential eligible receivers.
Now, when the offense puts themselves at a disadvantage by not having 11 players on the field, it was not uncommon that the "missing" player was (or should have been) a lineman. Only having six linemen because there's only 10 players on the field does not give the offense an advantage -- they don't get an extra receiver and they're missing a blocker. However, it was a foul on the offense. We'd march them back five yards because Bubba forgot he was in the game and let his QB get sacked because of a missed blocking assignment.
Philosophically, this doesn't make sense. Why would we continue to penalize the offense when they're playing at a disadvantage? The intent and philosophy of the formation rule was to prevent the offense from getting an extra eligible receiver (or six). Many officials recognized the absurdity of the situation, but we still flagged the illegal formation because that's how the rule was written. Only once the rule was changed to redefine the formation in terms of backs did our enforcement change. How many officials do you know that would have said "Well, I know the rule *says* seven on the line, but I'm going to let it slide because they only had 10 on the field"? We would have called them out for inventing their own rules and interpretations not in line with what the rule book says.
This is where I am with this whole topic -- I know it's absurd and doesn't make philosophical sense, but it's they way the rule is currently written and therefore the rule should be changed. Until it does, we have to throw the flag for "six on the line".
The difference in these two scenarios is that a nonplayer or UNS foul by R during a play with first touching by K where R would want to take the ball at the spot of first touching, rather than the result of the play, is exceedingly rare (at least in comparison to a six man line because A only has 10 on the field), so there's little momentum to address the issue -- or even awareness that there is an issue. Since it is so rare and unexpected, it's very tempting to go 1-1-6 on the play and do what you think feels right, but that's not in keeping with the rule book as it is currently written.
