So it seems you did make some changes that may have affected the tightness in your basement at around the same time you started having these issues? You don't say whether or not you opened a basement window as part of your "de-winterizing" though. Unfortunately you cleaned the vent and "de-winterized" at the same time so it's hard to tell if either of those may have caused the problem.
Bottom line to me is that by winterizing the basement it appears to me that you are trying to make it warmer and more comfortable there. That alone tells me you should install an OAK. Not sure if not having an OAK is causing your issues but the benefits of having one are many, especially if you are trying to eliminate drafts in the basement. You shouldn't have to take off your side panels and clean your vent pipe every two weeks to have your stove working properly. Good Luck!!
I've been cleaning the vent pretty frequently due to the black ash . There wasn't a lot of ash that came out, but I just wanted to make sure that there were no clinkers in the piping from all issues I've been having. We normally winterize our basement during the winters so I just did it by habit. I'm learning that with the stove down there I won't need to any more.