Modified prone presents a lot of challenges that have to be experienced to fully appreciate.
The brake, for example, will blow a divot in the ground and can coat you and your glasses and the rifle with grit. A linear brake solves that issue.
At Camp Borden, the scrub grass is on a very sandy soil. The brake blows the sand everywhere. I shot the same event on real grass at the Guelph club, and there was no reaction from the ground at all. In BC, modified prone would probably create a rooster tail.
You lay on your strong side. The “down” arm and hand are somewhat trapped. With a AR15, your hand would run the trigger and push the mag release. Left hand would change mags and push bolt release. No sweat.
With 858, the bolt release is on the down side of the rifle, so that gets a bit awkward. Reloading a SKS in modified prone would be very difficult.
When I shoot modified prone, I use my down arm to hold the forend, and I run the trigger with my top hand (my left). This makes for a lot less adjusting of the hold when I use the left hand to change mags. But this will only work if the gun allows the left hand to drop the bolt. Or, one could change mags after 4 shots, so bolt does not lock back.
This is what it looks like, when you switch hands and run the trigger with the wrong hand. As you can see, the support arm aiming the rifle does not get disturbed when changing mags. You can also see how a doubled mag makes a mag swap faster.