How do I reboot over a ssh connection without a return code of -1?

Use the shutdown command.

shutdown --reboot +1 "System is going down for reboot in 1 minute"

I suspect the reason reboot doesn't work is because it requires a tty. You could try running it with a background tty, but the shutdown command has everything you need, including cancelling -- as it says in response:

Shutdown scheduled for Thu 2018-02-22 15:19:33 MST, use 'shutdown -c' to cancel.