Just got off the phone with the vet at the lab. He gave me two (three technically) possible reasons why this bird passed (one of which could also be what the other rooster is dealing with).
1) Marek's disease: I'm hoping this isn't the case, because if so there isn't anything I can do.
2) Fatty liver / too much sugar (or as he called it, "energy"): Essentially going back to the feed problem, explaining the pale liver.
3) He could have some sorta infection, but as with option 1 I either need to do more digging (which I will) or I need to send in samples to either confirm or rule this out.
If it is idea #2, then that means that this guy had a fatty liver from the Layer feed, which caused his health to decline and ultimately flat line. Personally, I hope this is what happened. If so, then that is something I can handle by changing up the flocks feed. We are currently feeding them layer feed, with a bit of medicated start and grow. However, roos are not suppose to eat layer feed, as they do not need all the calcium and "energy". As a way to test this, we are going to have to keep our afflicted roo separated from the flock, and only allow him to eat "all flock" or any regular feed other than layer. If he improves, then we have our smoking gun. If not, then I'm stumped and will blame this on SDS (which is unlikely).
However, if he DOES improve, how do I go about changing the whole flocks diet? I thought about going for all flock feed, while giving the birds the option to eat egg / oyster shells, but would that work?
I am still debating on if I should send in samples, as I don't want to spend more money than I have to, especially if I am just going to get a confirmation that the food was at fault or that it IS Marek's. Like I said, I will do a bit more digging, and I will post more pictures here, along with any updates on the other roo, or anything I hear from the lab.
If I have seen anything in the process, it is that I am not the first to have such a mysterious death like this on my hands. I would hope that this process will yield some results that others can use to either understand what happened to their own bird, or to prevent it from happening in the first place. Thank you all so much for the tips and advice, and I will keep yall updated!
None of this explains the worm though...