I thought that third one looked lame but you are right it is clearly by the letter of the rule. I'm sure I wouldn't think it lame if the hit had resulted in a leg injury to Purdy as the danger of that is why that type of hit is illegal.
I thought a similar call on ISU earlier in the season was lame as well as the player was being blocked which resulted in him being unable to control his low hit on the QB. But again, letter of the rule.
I know the play you are talking about. There was 90 seconds left in the Okie State game. Benton got called for roughing. That allowed them to get their final TD. Actually this rule has some exceptions and one of them is that it is NOT roughing the passer if you are blocked into the legs of the QB. That is why the rule says it applies to a defender rushing unabated. So we likely should not have been penalized there. I'm not complaining but that was a subjective call and the ref appeared to maybe fail to see that Spencer was pushed by the offensive lineman which caused him to lose his balance and fall into the QB. He was not "unabated." But I think refs often err towards safety of the players, especially QBs.