What if my device never goes back online

There's nothing you can do to put it online. All you can do is wait and see if whoever finds it puts it online by turning it on in a location where it can connect to the internet.

If it runs out of power, that will keep it offline. If it's turned off, it stays offline. Under these and other circumstances, a device never comes back online.

You're welcome. You may be right, unless an honest person finds it, charges it and turns it on. If was stolen, it will probably be restored as a new device and you'll never see it.

My husband has set up Find my iPhone to send a lock command when my stolen iPhone is turned back on. Would the thief be able to restore it?

Yes, but if the phone went online and the lock command was then executed the thief would only be able to restore it as a new device, erasing your data.