Is it possible to allow sideloading and keep users safe?

Is it possible to protect users from themselves in every circumstance?

Yes. Remove all of the features from the software. Now, I know you're wondering, "What if my users eat the battery?"

Next, remove the hardware itself. Now users cannot harm themselves at all.

"If you are protected by a steel door, but you don't have the key, you aren't safe: You're imprisoned."

Unfortunately, some users manage to cause themselves harm using nothing but their own body.

Clearly, there should be a way to restrict their access to that too. Keep them from performing unauthorized bodily actions that could result in self-harm. For safety reasons.