Absolutely. When two sides meet like this, it's almost guaranteed that their takes on the situation will be a bit different. I have also seen people get into hearings with the employer producing write ups and reprimands that were NOT in their files when they got them. Believe me, it happens. Some employers who sound like the one you describe think they're smart, and they come up with all sorts of new reasons why they terminated the person besides the one they used originally. But yes, what he told you in the first place will definitely have a bearing on the hearing.
Don't worry, if this happens, if they "stuff" your file, the hearing officer does not necessarily have to have the actual file in their hand to determine that the write ups are not genuine. After all, if you produce the personnel records you've sent for, what's to say you didn't just take out the documents before bringing it in? There is always the expectation in these hearings that either or both of the parties may be lying.
What you do is stay right on your story. You state what you stated in your initial application for unemployment insurance, exactly what you were told by the employer at the time of your termination. If you get half a chance, work into your comments that you always did your job to the very best of your abilities, that you did not realize that your performance was inadequate and that you were about to be terminated. (Or if you were told and giving evaluations and warnings, be sure to state that you did your utmost to improve your performance and meet the employer's expectations.)
How this is looked at in unemployment law is that not meeting the employer's expectations, "poor performance" is not by any means always due to misconduct. They can set their expectations to the point that you simply cannot meet them. This is the "spin straw into gold" concept. If they ask you to spin straw into gold, you could try your very best and you do not have much chance of meeting their performance expectations. This is not considered misconduct.
If they hired you to do a specific thing, and it turns out you are incapable of doing what they ask, yet you have done your best to learn the job and do what they required, and you have not been able to please them, that's not misconduct. It may be that the employer's expectations are not reasonable. Or it may be that they have made a hiring mistake, and that they simply have hired someone who, even though they tried, could not do what the job required. If you are supposed to be able to perform xxx number of processes a day, and you because of your inherent slowness are not able to do that, then that's not considered misconduct. If you remind the employer of one of his uncles who was a jerk and he's always irritated by the sound of your voice, or you just do not fit in with the culture of the workplace, they can fire you, but that's not misconduct.
In fact, misconduct is rather difficult to show in performance firings. In order to show misconduct, the employer must show that you had the capability to perform the work up to their expectations, that you had, at some time or another, actually performed correctly, and then by choice or because of your behavior, have begun to fail to meet their expectations, and after receiving some feedback and warnings that your performance was jeopardizing your job, you voluntarily chose to keep doing it poorly or too slowly or not doing it at all, for example spending your work hours sleeping at your desk instead of doing the work. In other words, you had the choice to change or improve your behavior (and were capable of improving your behavior) and that you voluntarily chose not to do so, even though you knew it might result in your termination.
See what I mean? If you say you were doing your best, it is not misconduct to fail to meet performance expectations. Even more so if you did not receive quite a bit of constructive discipline and feedback from the employer, information about how you were not meeting performance expectations and given feedback that your job was in jeopardy.
What you do is tell your side of the story briefly, professionally, and truthfully. Do not argue with everything they present or say about you, do not try to rebut every argument they put up that you committed misconduct. Just stick to the theme that you did your job to the best of your abilities.
It's not necessary that you have your files to submit as "evidence" that you did not receive warnings. As I said, they go by who is most believable.
You have already won the first round. This means that the employer did not present sufficient proof that your termination was actually misconduct poor performance in the initial investigation. And do you not think that the appeals officer will be able to notice that they've changed their story and added a whole lot more offenses? In order to win in these circumstances, they'd have to be awfully convincing. Actual video of you sleeping under your desk instead of working would be a good convincer.
When making decisions, the appeals officer will tend to pick out the primary issue. Throwing a whole bunch of other supposed bad acts into the appeals hearing doesn't usually help the employer a whole lot. If they fired you for failure to meet expectations and then come in and start complaining that you also did not get along with co workers, or were always late returning from lunch or on one occasion were insubordinate to the manager, this is called a "shotgun" termination. They're throwing out a whole lot of things and hoping something will stick, which is not a good idea when you are trying to be believable.
You do not have to tell the appeals referee that your employer is lying about everything, that have known of your employer lying before or that you've seen them instruct other employees to lie. You don't have to go through and deny or even deal with each of these things, argue that you did get along with everyone, that you didn't come in late, were not insubordinate. Just state your position. This is what I did, this is what I was told. Be sure you keep it that you were told when you were terminated that you had failed to meet your employer's expectations, even though you ALWAYS DID YOUR JOB TO THE BEST OF YOUR ABILITIES. Simplicity and sincerity, not multitudes of evidence, legal argument or assertiveness, is going to make you more believable.

