The straight in shots are excellent at finding what a person is doing wrong. It helps if you have a measles cue ball at hand, too. If I see someone shooting badly with a measles ball I can usually tell what flaws they have after about 5 mins watching them hit straight ins.It seems to me, if you were missing that badly, it had to be your vision. I take everything you say with a grain of salt however.
I would have set up a simple straight in shot to start to diagnose the problem. One diamond from pocket, one diamond between OB and CB. Shoot it. Did it go straight in center pocket? If so, move CB back 1/2 a diamond. Repeat until you miss. When you miss, start tweaking some things to see what takes you back to center pocket. Could be you came in and had unconsciously altered your head position. So you're aim is skewed. Perhaps you're rolling your wrists. If, as you say, you were missing all the shots by 1/2 a diamond, that is a lot. Should be easy with a few simple diagnostics to see where that is coming from.
Of course, you were the guy that was going to video yourself routinely hitting 5 bank shots in a row after starting with ball in hand, on a diamond 9 footer. How you can do this and then come in and be off 1/2 a diamond is puzzling.
Set up a close straight in, OB centre table, CB 1ft away and shoot into a corner. Hit the shot firm. If the ball has any rotation, top, bottom or aide spin then you are doing something wrong. Likewise if the CB doesn't stop dead, but goes along a tangent line then you are also no doing something right. If its going off on a tangent, but has no side spin then its an aiming/ sighting issue. If it has side spin and/ or goes off on a tangent line then its a stroke fault or a mix.
But yeah, missing by half a diamond is beginner, and complete beginner standard at that if it was happening a lot. Are you sure you were shooting with the correct shooting hand? ;-)