There is a difference between being reliable and being observant. Greer admitted he had only vague recollections of much of what was going on around him after the first shot.
Reliability is determined by fitting one's recollections to the rest of the evidence. If you wanted to establish whether there were people on the overpass, you would not use Greer's evidence because he admitted that he had only a vague recollection of that. But if you wanted to know whether his clear evidence about when the first shot occurred, you would see how it fit with the rest of the evidence. As far as I can determine, it fits very well with the rest of the evidence.
According to Greer, the limo was just about past the western edge of the TSBD. That puts it within 20 frames of z200, either way. That fits with a host of witnesses along Elm St., witnesses in the motorcade and photographers such as Phil Willis and Hugh Betzner who identified the time of the first shot in that same range.
Greer also had a clear recollection of turning around immediately after hearing the second shot, which he does at z278-80 and seeing JBC falling back. We can see that in the zfilm. The recollection of a shot just prior fits with what Hickey and Kinney observed with the hair flying up as seen in the zfilm at z273- 76. It also fits with the shot pattern recalled by over 40 witnesses.
This is quite a nifty side-step of the copious amount of evidence presented to demonstrate how poor a witness Greer is. That he is unobservant cannot be denied, this alone puts a very large question mark over his ability to place where the first shot occurred, particularly, as he noted himself:
"Well, when we were going down Elm Street, I heard a noise that I thought was a backfire of one of the motorcycle policemen. And I didn't--it did not affect me like anything else. I just thought that it is what it was. We had had so many motorcycles around us. So I heard this noise. And I thought that is what it was."The unobservant Mr Greer never really paid any attention to the first shot, let alone his position on the road at the time. But, as you say, there is a big difference between being reliable and being observant (unless, of course, it comes to witnessing a crime), so let's use your own definition of reliability - "determined by fitting one's recollections to the rest of the evidence".
Greer states that he was focussed on who was on the underpass. When pushed he said one officer (perhaps):
Mr. CRAIG. Do you believe if you had observed people on the overpass at that time you would now remember it?
Mr. GREER. Yes, sir; I believe I would; yes, sir.The photo Jerry posted shows 10 men on the underpass. 10 men!
Greer is clear that he turned round one time, after hearing the second shot, then hit the gas.
The Z-film shows him looking round twice.
At no point does Greer state that he hit the brakes slowing the limo down. He doesn't even hint at it. He gives the distinct impression that he did not touch the brakes:
Mr. SPECTER. What is your best estimate of the speed of the car at the time of the first, second, or third shots?
Mr. GREER. I would estimate my speed was between 12 and 15 miles per hour.
Mr. SPECTER. At the time all of the shots occurred?
Mr. GREER. At the time the shots occurred.
It is known from multiple sources that the limo slowed quite dramatically after the first shot but Greer recalls something quite different.
These examples alone demonstrate Greer's unreliability by your own definition but there's something more - I'm starting to become really suspicious of Greer. In your post you state:
"Greer also had a clear recollection of turning around immediately after hearing the second shot, which he does at z278-80 and seeing JBC falling back."
How clear is his recollection? Firstly he turned round twice, something he fails to mention. In the Z-film, the second time he turns round he appears to be looking in the direction of JFK at the time of the headshot, something else he fails to mention. On closer inspection we find Greer lying about his observations when he turned round:
"Mr. SPECTER. And describe or indicate how far you turned your head to the right at that time?
Mr. GREER. Just so that my eyes over, caught the Governor, I could see,
I couldn't see the President. I just could see the Governor. I made a quick glance and back again."
Mr. SPECTER. Were you able to see anything of President Kennedy as you glanced to the rear?
Mr. GREER.
No, sir; I didn't see anything of the President, I didn't look, I wasn't far enough around to see the President.
Greer is adamant he never saw the President hit even though the Z-film shows him looking in that direction at the time of the headshot. Then later on in the same testimony:
Mr. SPECTER. And the first part refers to your noticing that the President evidently had been hit. Did you ever
Mr. GREER.
I have no recollection of ever telling the agents that I said that; no, sir. If I said it, I don't remember saying it. The Governor was the person that I knew was--when we were first in trouble, when I see the Governor.
Mr. SPECTER. To the best of your current recollection, did you notice that the President had been hit?
Mr. GREER.
No, sir; I didn't know how badly he was injured or anything other than that. I didn't know.
Mr. SPECTER. Did you know at all, from the glance which you have described that he had been hit or injured in any way?
Mr. GREER.
I knew he was injured in some way, but I didn't know how bad or what.Mr. SPECTER. How did you know that?
Mr. GREER. If I remember now, I just don't remember how I knew, but I knew we were in trouble. I knew that he was injured, but I can't remember, recollect, just how I knew there were injuries in there. I didn't know who all was hurt, even.
Greer denies seeing the President hit four times before admitting he knew he was injured in some way and when asked how he knew the President was injured he comes up with the quite awesome phrase "If I remember now, I just don't remember how I knew..."
This last statement of Greer's does not speak of reliability.
As for the slight ruffling of JFK's hair being the same as his hair "flying up"... I'm not so sure. I think the observation of JFK's hair flying up seems more like when part of his head flew up in the air with hair attached to it. Other witnesses make the same observation about his hair flying up but always in reference to the headshot.