We only had a glimpse of the autopsy report, we don't know if any damage occurred to the head or not. But your argument doesn't make sense because of the damage to the mask, there should have been bruising to the head - that would be the case whether or not Shelley ripped her mask off or Swain ripped her mask off. You are really arguing that the mask was not ripped off of anyone or that the mask was never damaged at all. The picture of the mask where the strap threaded through and where the missing pin appears does not show any cracks, so there was no pre-damage like you tried to assert earlier.
First of all, I didn't "try to assert" anything, I just asked a question. I raised a possibility. Since the pin is missing, we don't know what state it is in. The only possibility left for a spontaneous failure would be that the ends of the pin were rounded too much such that it wasn't as tight a fit as it should have been. That's incredibly unlikely. The mask doesn't look like the material is particularly fatigued or out of alignment in a way that would make it easier for the pin to come out.
I could much more easily see that Shelley could have done that damage to her mask without leaving a mark on her head than that David could have. That's because I know that I could put a lot of pressure on the strap pin mechanism on a mask I was wearing, or on a mask I had momentarily pulled off to adust, without bruising my face. I do not see howanyone could do that to me against my will without leaving one, though.
Looking at the mask, I don't see a picture of the strap. Was it really broken? It would seem odd to me if the pin gave way that the strap would break as well.
Ayisha's claim was:
Mask strap broken on left side.
Mask pin missing on left side.
Snorkel holder (on left side) is broken.
Part of snorkel tube (on left side) is broken off.
Snorkel mouthpiece (on left side) is missing.
My recollection was that there was a claim about the strap being broken, about the snorkel being disconnected from the strap, and about the mouthpiece being disconnected and lost.
I neither recall nor see any damage to the snorkel tube. I don't specifically recall any mention in the various reference materials that the snorkel holder, whatever that mechanism was, had broken. Many attachment methods would readily fall off a brokem or disconnnected strap without being damaged. That said, I don't see any holder mechanism in the picture, so I would need more evidence to form a belief.
Based upon the above, I believe it is possible that there are only two points of damage here. First, the missing pin, and second, the missing mouthpiece.
If one looks carefully at the mask, you can see that there are channels to assist in getting the missing pin installed. You can see that, as part of installing or removing the pin, the upper and lower parts of the mask in that area would have to be slightly flexed away from each other in order for the rigid pin to be seated.
At that point, the pin could be removed in one of two ways. First, and I think the way one would do it for maintenance, would be to press apart those extensions of the mask to allow it to fall out. Second would be to apply enough horizontal pressure to the pin for it to force the holding plastic out of the way and pop free. The channels provide a weak point for such force, but they go out to the sides. In use, the strap applies pressure approximately straight back from the face, and I believe the pressure required to break the pin free in this direction would exceed the pressure that a rubber strap could tolerate and the strap would break first leaving the pin in place.
Based upon the above, I believe that the strap would either have to be pulled straight to the left side roughly in line with the channel in order to have a chance to pull the pin out before snapping. Either that, or pressure had to be applied directly to the side of the mask body to separate the wings allowing the pin to fall out. The latter is obvious, as it is the way the mask was designed. The former is probably testable, and I think the responsibility of both sides for their own reasons.
Based upon the above, there are a few scenarios that work. If Swain was part of it, there are a few scenarios. He could have grabbed at the side of Shelley's head during a struggle and the mask pieces could have been spread. He could have grabbed for her mask and started to pull it free. If Shelley responded quickly enough to this, there could have been a tug-of-war for the mask that resulted in one pulling sideways on the mask body while the other yanked on the strap or snorkel. Going with the driven-into-the-bottom theory, the left side of her head could have been pressed down into the bottom in such a way as the mask wings spread enough to free the pin. The latter two explanations also extend to account for the missing snorkel mouthpiece. If Swain didn't have a part in it, the scenarios I see are that Shelley stuck her head into a wreck and caught her snorkel on the hole, more likely while pulling out. This might have pulled the mask off her head and a clumsy attempt to pull it free from the wreck could have led to the loss of the mouthpiece inside the wreck where it might not have been found. Pulling the mask sideways, which I think we would be likely to do in this circumstance, could explain the pin popping free. Other than a very odd panic episode or someone deciding that 90fsw was a good location to practice tearing down their equipment, I can't think of any other scenarios that explains that part of the evidence.
Evaluating those possibilities on their own and without further evidence or testing of the scenarios, I could not say that it was Swain beyond a reasonable doubt. K_girl, you have said the mask damage is pivotal for you. For me, it is the fin. I can't come up with any reasonable scenario that fits a fin in the sand with the most plausible mask scenario not involving Swain. The best I can do, and it's pretty far fetched, is that somehow Shelley ended up with her foot in the sand and her fin came off. She thought this was really cool and went to look for her husband to show him, not realizing that he had already finished looking at the wrecks and headed off to the reef. She left the fin there and went to find him, sticking her head into one of the wrecks as she looked around. Her snorkel caught as she pulled back out and her mask started to come off. This panicked her slightly and she tried to tug the mask out, breaking the snorkel mouthpiece free and releasing the strap pin. This escalated her panic and she was lost. That's the best I can do. I'm not sure if the evidence could prove it to be impossible, and either way, it would be a stretch to get a jury to accept it as reasonable.
Now, don't attack me for trying to assert this is what happened. I'm not. I'm just putting forth a possible scenario.