I do believe total work in kilojoules is listed in the workout summary at the end of the ride. I can’t remember for certain at the moment though. As for why Zwift doesn’t display calories currently, I couldn’t tell you. I suspect it’s something they don’t deem that important for whatever reason. It’s one of those things that some people will care about, some won’t. It just depends on what your focus is. It’s also very open to interpretation. There is an approximately 1:1 relationship between kJ and calories, but how close that relationship is depends on who you ask. It comes down to the efficiency of the human body and it’s way too deep a subject to go into here. The folks at Zwift may have just decided to stay out of those murky waters and let you use your platform of choice to determine calorie burn. If so, it’s a decision I can certainly understand.
The customization stuff is mostly unlocked as a result of “leveling up” or completing challenges. As you ride you accumulate points for workouts and mileage (or kilometers, whichever is your preferred unit of measure) and as you accumulate points you reach new levels. When you hit levels different things are unlocked, be it wheelsets, bikes, or kit. Additionally, completing either of the two challenges unlock gear as well. Titanium Geek’s unofficial user manual lists all the unlockable stuff.