If pilots have no control over brakes / air brakes / reversers, can they slow down the plane after landing by carefully steering it off the runway into the grass? Would it be better to put only one gear into the grass or all three?
Depends on what the alternative is. Hopefully you're going fairly slow when you get to that point. In any case, if the choice is between grass and a drop off into a ravine, go for the grass. If the choice is between grass and steel lighting structures off the end, grass wins again. If the choice is between grass and special overrun material like sand or EMAS, well in that case grass loses.
How you do on the grass will depend on the surface condition, how dry it is, etc and how fast you are going. If you're going fairly slowly, you won't likely collapse the gear, just make big furrows. I would prefer to get both mains onto the grass to minimize the risk of going sideways and collapsing the gear, which is what you really want to avoid.
But in any case, you will do better on the turf than going over a cliff or into structures that are going to shred your airplane even at low speed. If you can see what is beyond the end of the runway as you get there, the choice will be fairly easy.
Not a good idea.
Airliners are heavy objects with a lot of weight on just a few wheels. Airport soils are generally pumped for low ground water level, if needed, to make soil firmer. I know Amsterdam airport Schiphol is. Then still, the grass is in most cases too soft to carry the load, making wheels sink in. This will most likely slow down the one side that goes off first so much that further directional control cannot be maintained, and the aircraft crashes. For this reason, even aircraft specially designed to land on grass would do better to stay on either the grass on the runway during a landing.
Other than that, it's like with belly landings. The best place by far to land an aircraft is on a runway.
An increasing number of airports is equipped with a runway extension composed of material specially designed to safely stop an aircraft after overrunning the runway.
Typically, you have a main braking system, an alternate one and a backup one. Then you have reversers, and runway over run.
The only time i would choose grass (in an airliner) is the one at the end of the runway.
Also, the runway is a better platform for emergency vehicules and passenger evacuation.