Hi Andy, I use a wheelchair too (Puma 40) and had similar problems with carrying my gear. Hanging stuff off the side is a non-starter as you know because you're screwed every time you have to go through a door. Stuff hanging on the back is difficult to get to, makes the chair unstable and is asking to have it stolen.
I'm afraid have no experience of the Magnum other than seeing a review on YouTube, but it looks like it may be heavy and uncomfortable if you intend to carry it on your lap.
I use two bags depending on the load I want to carry; the first is a Lowepro Inverse 200 AW - this sits on my lap nicely and will carry my camera, a spare lens and a flash (or third lens). The second is a Lowepro Flipside 400 AWII - I carry this on my chest and when I unzip it, it opens down on to my lap for easy access (the zip is on the inner part of the bag). This allows me to carry the camera with a Sigma 150-600 attached as well as carrying anything else I'm likely to need.
One thing I should mention, is that with either bag it can get a little uncomfortable and sweaty - especially if you load them up. The same goes for everyone carrying a bag I know, but obviously we can't relocate them like an AB person can.