It is my view that there is no reason for Rudy to go downstairs. It would be incredibly stupid to hang around the cottage containing a dead girl. The boys downstairs say the cat was injured and there was already blood in that apartment before they left for holiday. The tracts are consistent with the path that a cat would take.
You are presenting a conspiracy theory. For your theory to be true, who would be involved in keeping the secret? What do they have to gain from this action, what is their motivation? Start with Rudy. He tells us in the chat and Skype conversation all the things he did upstairs that would leave evidence of his presence down to drinking from the juice container in the fridge. Does he think nobody is going to notice his blood downstairs?
The question isn't whether
you think Rudy has a reason to go downstairs. The question is, whether there is any evidence to suggest he did so.
(btw, might needing a change of clean clothes after a bloody murder that would fit Rudy (downstairs guys would more likely fit, girls less likely) so he could try to go home in cleaner clothes, could that be a reason for Rudy to go downstairs? To not get busted in bloody clothes from a murder scene on the way home?
As to how rational or straight thinking Rudy might be, seems like a lot of assumptions for someone with plainly observed mental problems, like experiencing black-out and fugue states, as Rudy reportedly had (Nina Burleigh went into this at length in her book, not sure if you take the Grinder note on this one.)
The point is, your expectations can be a great guide to help interpreting the evidence, but it can't outweigh the evidence.
The DNA experts (maybe you're one as well) on the site can argue the point more persuasively than I, but there were several human DNA profiles from downstairs that came back with human DNA, including on a light switch. (How can a cat drip blood on a light switch?) Those results have been suppressed.
Your assumption is the downstairs DNA can be explained as environmental. Perhaps that's possible. Were all of the samples upstairs also testing positive for DNA? Seems like we need to see the results before drawing a firm conclusion.
If the downstairs DNA turns out to be Rudy's profile, would you still deny Rudy went downstairs because you don't think he had a reason to go downstairs?
You don't see any reason for the police to conceal the downstairs crime scene and to protect Rudy early on. Fine. I do. I think there's support for my position, hence I hold it. I don't think I can convince you on this point, from previous exchanges.
But the question of conspiracy, and towards what end, etc, these are all separate questions from the evidence. There could be evidence of Rudy downstairs, yet the facts fit together in a way I'm not seeing.
Establishing Rudy being downstairs is only establishing Rudy being downstairs. I think an argument can be extended from that eventuality, but there's no logical need to jam every element in up front in an analysis, before considering a single fact in isolation. Hope I wrote that as intended and you're getting my meaning.