Also, just because it hasn't been set yet, disposals are activated by an electrical switch, typically built into a wall near the sink. They aren't activated by pressure or anything like that. It's not likely that they would turn on unintentionally (outside a Final Destination movie). You can safely fish objects out of them when needed, even manually rotate the gears/blades if something is stuck.

I reach into mine all the time, especially if something falls in that doesn't belong. I have the same brand, "In-sink-er-ator," and it's only about (EDIT) 20 cm deep. The "blades" aren't sharp, it's the speed with which they independently rotate, catching the food between them and battering it to pieces. There's a rubbery flange to keep stuff from flying out, you can remove it to clean or see if you dropped your wedding ring in there. It won't grind unless you flip the wall switch, which is in reach so you can turn it off if you're stupid enough to get your hand in it while it's going. It could cause serious injury but you'd survive.

Plumbers make a lot of money unclogging drains of people who forget that even though the garbage disposal can grind something, it can still form a clog down in the drainpipes.