IMO, you're narrowing the scope of the type of player that you believe the Kings need to add in order to make the point that they have a gaping hole in the roster that only you seem to see.
Vezenkov is strictly a stretch 4 and I view Lyles as a 4/5 but I'd call Barnes and Murray 4/3s. Especially Murray. If he's a "stretch 4" and not a wing and you already grouped 2/3 players into a different category, then I'm not sure who fits in this wing category other than apparently guys with 6'11" wingspans who can't shoot from outside consistently.
KZ Okpala started two games at SF for the Kings. Then he was replaced by a rookie because Murray gave the team a better chance to win.
Kessler started 3 games for the Kigns, all at the SG spot when Huerter was injured.
I'm sure Mike Brown would love to start a defensive stopper at a wing spot, but that player would have to be better than Huerter/Monk, Murray or Barnes and that hasn't happened. Mostly because the offense was significantly worse. Very small sample size, but the Kings lost both games that Okpala started and 2 of the 3 that Kessler started.
But back to the idea that it's unlikely that Prosper is better than Kessler Edwards next season. Even if that weren't the case, I don't see a scenario where both of them play. You can't have two defense first non-shooters on the wings and hope to keep pace when the starters are out. So if only ONE wing stopper is going to get significant minutes off the bench and the Kings already have one of those players, how is that the team's greatest area of need?
The only way to make that claim is if the type of player you want is better than Barnes or Murray. Prosper certainly won't be this season and very possibly will never be. The Kings weren't going to pay Grant $30M a season, and even if they were there's no guarantee he chooses Sacramento over Portland.
Cam Johnson or Grant Williams might have been minor improvements over Barnes on defense, but both are still "Stretch 4's" and not 6'11" wingspan switchable defenders based on your criteria. So I'm not sure who (a) fits your parameters (b) is better than Barnes and/or Murray and (c) could have reasonably been had this offseason.