While I despise the Cowboys, it seems like the mishap (to the extent it was a mishap, which appears highly likely given the video evidence) was detrimental to both the Cowboys and the Lions. Obviously, if the ref misunderstood who was eligible, he would have told the Cowboys' players the mistaken player (70) was eligible, and the Cowboys likewise would have understood that 70 was eligible (and likely not even attempted to cover 68). This wasn't the refs "screwing" the Lions. Unless the ref told the Cowboys 68 was eligible (which appears highly unlikely), if they didn't call the penalty, the Cowboys would have been "screwed over" by the officiating. This appears to be a case of the ref making a mistake, and who knows how the play would have unfolded had the ref gotten it correct.
Also, I heard it mentioned during the broadcast, but haven't seen it really discussed...wouldn't the Lions have been in an illegal formation had 68 been eligible?
Regardless, the Lions had two additional attempts (including a "free play") to get the 2-point conversion. In other words, they had their "opportunity" and didn't take advantage.