Puck JS - Advertising Beacons/Analytics Platform

how hard will it be to write the code for it?

It depends on the sensor, but it should be pretty easy. For instance there's an example in the Making Things Smart book that changes the URL when a door is opened: https://github.com/espruino/making-things-smart/blob/master/experiment35.js

95% of that code is for the sensor, with a single line (require("ble_eddystone").advertise("goo.gl/D8sjLK#"+doorOpenings);) for the beacon.

2) Can the Beacon's short URL advertising message be updated/changed via web site anywhere in the world at anytime etc, or do we need to bring the Puck JS back to the office to change the message each time/day etc.

Yes and no - you could advertise a link that goes to a website that you control that redirects elsewhere. Since Puck.js has no internet connection you can't reprogram it remotely though - have a bit of a read through other tutorials about beacons online for a bit of background.

Can an end user also change it if they want very easily via the web or an App for example?

They could use a web app to reprogram it, yes. You could even make your own Web Bluetooth website that would do it.

3) Can we give to customers who rent/buy our Beacons stats back showing them how well or how bad their Beacon advertising message is working?

This is general beacon stuff - but in the most basic form you could use the goo.gl URL forwarder, and that can provide you with some stats about usage.