The lamp is paired via ANT+ and is therefore compatible with Garmin products. It can also be used as a stand-alone lamp without a Garmin head unit, but some smart functions such as automatic brightness adjustment only work with a compatible Garmin device. Whether it works with other velocomputers depends on their ANT+ support, but the smart functions are specifically optimised for Garmin devices.
Yes, it is permitted in Switzerland, with the "daytime flashing mode" the energy consumption is very low and a day trip is no problem. Daytime flashing mode is not permitted in the EU.
I'm not sure, but I believe that the more expensive unit comes with a handlebar mount, the helmet mount is just a "strap for the helmet", I subsequently bought a Harmonie mount that holds both the bike computer and the Ut 800. Very handy! If that is included in the price, that would be an explanation....
Unfortunately, I couldn't find this information anywhere at the manufacturer's or on the web in general. With a bit of luck, someone else will answer your question.