The reason we don't really know who makes the final call on transfer decisions is because there is no single person. It's probably discussed between CEO and Manager mainly, with either having a certain degree of veto power, but owners have ultimate power, though they don't get involved as much.
Suggesting it's either Woodward or Mourinho to blame for the Sanchez transfer is silly. 80% of the fanbase were optimistic and feeling very happy about getting Sanchez.
If it was purely a Woodword signing, Mourinho wouldn't have to play him, so there has to be some level of agreement over transfers between manager and CEO, particularly with a club like United, without a good structure or DoF. There is no other way they can function without transfers. Although "functioning" is a bit of a stretch in describing United at the moment.
At the end of the day, 100% of the blame is down to Sanchez. It's obviously not intentional on his part to be such a dismal failure for United, but there's not much more that needs to be debated regarding Sanchez. United undoubtably should bite the hand off of anyone willing to take him at this point. Worst transfer in United history.