As you say, you don't really need something like the jamhub, which provides multiple headphone outs. You're correct that all you really need is a small analogue mixer - like the one you linked to, but there are many, many models available.
If your guitar amp really only has a speaker out, you won't want to plug that directly into the mixer, as it will be ...
If you want just a single monitor mix for IEMs for all band members, you can assign all the X AIR buses to the headphone jack and plug a headphone splitter amp into the jack. Not individually configurable but only costs about $30 for everybody, not including the IEMs.
Reference: In Ear Monitoring with the Behringer X-Air XR18
tl;dr: There is not really any alternative.
The Behringer P16 Personal Mixing System is literally 10 times cheaper than the competitors. The main competitors are Aviom and the Allen&Heath ME system. For Aviom, just the injector alone (i.e. the component that takes analog inputs and encodes it into Aviom's protocol) is 1500€. The Behringer equivalent, ...
@YourUncleBob is of course correct. Will accept the answer if posted.
Realized though that the reason I felt like my colleagues' settings were "backwards" was something totally unrelated, and kind of interesting and a good lesson for beginner DJs.
tl;dr - Yet another reason not to rely on visual cues!
It turns out I didn't have a different setting than my ...