How can I monitor usage of an iPad that is probably stolen?
If the isp has no data, you aren’t likely to get any better alternate than Apple lost mode / find my iPhone and pings from the cellular network. 

If you truly think it’s stolen, consider filing a police report and send that to the carrier and ask for any pings. 

Your reasoning is sound about the battery dead situation. In a few days, you’ll likely never hear again until someone powers it up or charges it near WiFi or cellular coverage.