It's true that the games without Lillard are a small sample size, but I don't think you can so quickly dismiss that a player might well have a different mentality as a starting point guard as opposed to a starting shooting guard who sometimes slides over to the 1. In the second case, McCollum could still very well see "shooting guard" as his identity, even if he's the main play-maker for a reserve-heavy squad.
Looking at a larger sample size, for his career he has an approximately 17% Assist Rate. That's nothing spectacular, even for a shooting guard, but it's also not the Assist Rate of a black hole. Good play-making guards would probably be in the 20-25% Assist Rate neighborhood. Andre Iguodala (one of the best passing swingmen of his generation) has a career Assist Rate of about 20% (but was between 23-26% in his prime). So McCollum isn't exactly an Iguodala, but he's around what Iggy was doing in Golden State (and Golden State Iggy was hardly a score-first player on offense, but he used fewer possessions and was past his prime, which allows McCollum to catch up).
To sum up my view, McCollum isn't a natural enough passer to be a long-term point guard but he's a much more willing passer than he's often given credit for on this forum. Yes, he shoots a lot, but he plays most of his minutes beside the ball dominant Lillard--a lot of his role is to score off Lillard's actions. I don't think his game is selfish and I think that he's at least a competent passer. He can make the normal reads and passes, but not the step-ahead reads and passes.
