I don't have a dog in this fight, but suggesting there was some kind of demand or mandate to take a certain player seems awful counterintuitive. GM's are responsible to get players the coaches want, seeing as how almost every coach in the NFL has control over the roster. The scouts typically work for the GM and provide their insight on the players. The GM and coach must work through the selection process together based upon their wants and the information provided to them. An owner telling a GM and coach who to take, despite all the work those two put in, not to mention the work of their entire scouting department seems far fetched even for a completely dysfunctional front office.
I really don't know much about McNair, but I know from interviews I read when I first started watching the Texans and reading information from his Dad, Bob, that he felt the same way about the relationship of the GM and coaches. Agree that they may say, "We need to take a QB with our first pick", but to demand a certain player seems a little off to me when they spend a buttload of money for those guys to make those decisions.
I could be wrong obviously and maybe the Houston front office is just completely F'd...