I'm sorry to have to contradict you, but very little of what you're saying here is accurate. A few particulars, without going through every line item by item, to make sure the understanding gets set straight of how these things work.
"1. Without the tape itself, no one can judge what is actually on the tape."
That is way off base, and a misconception of what evidence is.
If you don't have something (in this case, a legendary video), and it may no longer exist or may be unavailable to bring to court for some reason, then an alternative is to find people who have seen it and are willing to testify as to the details of what they saw.
"4. The case against Mark Pearcy is even more problematic in legal terms. It is based on hearsay...actually, hearsay within hearsay."
That's way off base as well. If someone says they heard his voice, then that itself is evidence (eyewitness, or maybe you want to call it earwitness, testimony). The person testifying is stating what they personally observed.
Hearsay would be if I saw the video, and told you I saw it, and then you testified that there was a video.
"Without the tape, we can not even judge if it is evidence of a crime"
Yes, you can. You can get the testimony of someone(s) who saw it, and ask what it showed.
Overall. the questions you raise are not questions of admissibility as evidence. Instead, they are things that come down to the testimony and cross itself, and are ultimately what a jury decides, as to how much weight to give things. If they believe her, and are convinced she was dead-on accurate in everything she said, they don't need a second person to be able to say the same thing, or the tape itself to back her up. BUT - the jury gets to decide what's good testimony, what's weak or needs to be ignored, what's enough, and what isn't.