Fantastic! You have intriguing insights: “…it seems easier to achieve a sympathetic oscillation using this less-than-passive exhalation, because the aperture is smaller, which means there is less lip tissue oscillating. But this may also be taken as a hint that my mouth and/or aperture corners are too tight to allow a broader portion of the lips to oscillate freely.”

What is a sympathetic oscillation? I have a guitar in my room. When I play pitches on my trumpet that match the pitches that the strings are tuned, those strings vibrate without me touching them. It is the soundwaves’ energy that sets them vibrating. When Greg discusses the sympathetic oscillation of the lips, I believe he is referring to the leadpipe air-turbulence interacting with the lips to set the lips and the pipe vibrating together. Sympathetic oscillation requires two objects to interact, in this case, the leadpipe and the lips. The medium of this interaction is air reacting similarly between the guitar strings and my trumpet. All that is needed is air to be released through your embouchure with the mouthpiece and leadpipe system. No blowing or “less-than-passive exhalation” is required. It’s the slingshot and release. Your conclusion could be spot on: “But this may also be taken as a hint that my mouth and/or aperture corners are too tight to allow a broader portion of the lips to oscillate freely.”