The guns Dom's, Dean picks it up after being told by Thanatos and just leaves it lying around in his room. The knife is Oz's you can ask him about it during the questioning. Oz says he just left it lying around the mansion but doesn't know where it is.
Correct, Dominic's gun is also in play though at what point would Roswell be incentivized to go and retrieve it? We know from the sequence of events that Benson's movements insofar as letting Hoss into the library, back through the passage and upstairs would imply that he was the one pursuing and not the one that had been ambushed. With this in mind, the rationale for Roswell to even consider getting the gun falls apart, unless you can find another reason why he'd have a motive to do any of that.
As Cerberus said, the knife was Oswin's. And as we see from the crime scene, the way in which he was killed doesn't line up for someone like Jack who is not the best at stealth from the amount of branches he supposedly steps on through the woods when Dave is around. Additionally, he has at least two knives of his own; why would you suddenly swap weapons when you have no hesitation of using your own as-is?
Then I suppose the final sticking point is the Benson v Memphis match-up. We see already from the bad path ends that Memphis isn't too concerned with Benson when they confront one another in the mansion, which gets them their escape after Benson threatens to shoot Oswin instead.
I'm not saying you're wrong, I'm not saying you're right, but your reasoning is fundamentally flawed based on what's in-game already.
Also hi.