knife could have belonged to Kaylee or Maddie.....which the sheath did have female DNA on it.

That is not known. We know the snap/ button did not have anyone else's DNA. So if it belonged go KG/ MM they didn't open/ handle it? If the sheath/ knife belonged to KG/ MM it is still (indeed, more) incriminating for BK's DNA to be on it, as it further limits chances for innocent contact or transfer of his DNA to the sheath if he has no connection to the victims.

up to the prosecution to prove where the knife came from

That is a different point. The sheath under a body with Kohberger's DNA on it is incriminating, irrespective of whether the knife can be traced to his ownership. Also if the sheath not being owned by MM/ KG or roomates/ bfs etc is established. In many murder cases the murder weapon is not recovered, that does not prevent prosecution.

even a planned multiple murder - it doesn't mean they bring their weapon with them

I'd guess most planned murders involve the killer taking the weapon with them. Perhaps more unplanned, spontaneous killings involve the killer grabbing a weapon of opportunity at the scene already.

sheath might have already been there prior to the murders

That makes little sense both in terms of only Kohberger's DNA being on it and it being in the bed.

the whole sheath had already been pre-cleaned of all previous DNA

Which is getting into a well planned framing plan. In which case, given that Kohberger had no felony conviction and his DNA was not in CODIS, his DNA may not have been useful. Your "framer" assumes there was sufficient (and time stable) DNA on the sheath; your "framer" assumes the DNA will lead to Kohberger even though he has no record; do you think the framer anticipated successful use of IGG which, certainly to a lay-person, assumes a Kohberger relative had done a genetic genealogy test. Why does this "framer" who goes to the trouble of sterilising a sheath and handling it to keep it sterile other than BK's DNA not plant BK's hair or some other item, and why did the framer not phone in a tip on Kohberger? Seems a high risk, high uncertainty, odd framing full of assumptions. The match of the eyewitness ID to Kohberger, the match of his car and (likely) match of his unusual shoe size also seem incredible coincidences or very difficult for a framer to plan and stage also?

to be driving around in the area that night, had their cell phone off at suspciious times>*

Well, this is looking to be a lot less certain right now

What is less certain? Even his own alibi confirms he was driving in the area at the time, and his phone data shows him c 5 miles away to the south a short time after the car sped away in that direction from the scene.

Rhodes scholar, not "road" 

I wondered about "road scholar" and thought maybe as Payne has "eluded" elusively to so many streets and highways in the PCA it related to that and not Cecil.

posted on here about the Canadian mass knife murders, which that was a planned mass murder

I think that is a really bad example, as the killer there was having a mental breakdown, thought he was turning into a vampire and had stuffed garlic into his socks - it was not a rational, planned attack, but more a result of deteriorating mental condition at the time.

If the sheath was already there, why was noone else's DNA on it?

I agree that killing 4, and EC being there, may indeed have been unplanned and XK being awake a surprise to BK.

Yes, i'd guess guns would be most, but there are also knife attacks that leave no DNA

Probably difficult to find that breakdown of data I'd think.