I read all and INMHO this is the best solution:

A daemon running in background embedded in the OS, checking periodically (hours, days, can be setup by user) an address, sending IMEI, receiving answer. If answer is not OK then starts GPS, sends coordinates by any means.

I doubt that the thief will hold this phone off. Even if he will sell it, the buyer can be tracked by police and you will get your phone back.

But this must be enabled by default on the OS, not a daemon running or installable app. That will be very easy to turn off. And don't think that thieves today are ******s... they know very well what they are doing and this phone is not for anybody. The guy who will buy this from a shady person will for sure try to turn the feature off.