The "light leak" shown in those videos looks like it might be two separate things, first, lens flaring when aimed toward a strong light in an otherwise dim room, and second, an internal reflection from something inside the mirror box when the camera is aimed toward a bright source that strikes the lens front at a steep angle. Looking at it the Fred Miranda pages, the most likely issue seems like it's the bottom edge of the mirror--which appears it would benefit from a swipe with a black magic marker. If the camera had "a light leak", it would be ruining actual photos. lol.
Tough choice between ditching one complete outfit with good lenses, but a camera that has burned through two shutters and is on its third shutter, for almost an identical outfit with good lenses and a VASTLY better ultra-wide zoom, along with a newer, better body with better AF system and better image quality. Ditching never feels really comfortable. You have the money and the experience tied up in the Canon setup, but the AF in the Mark II was always the limiting factor. Blown focus and not being able to shoot unless stopped down enough to cover focus areas sucks. If you can find a way out from under the Canon gear and not have to take much loss, that would be a great situation. I think you'd also realllllllllly like what can be done with a Sony-sensor-shot file when it comes times to shoot against the light, and you need to lift the shadows, and not have noise and chroma pollution in the shadows, even at base ISO.