For me, the hardest part is that you can't befriend upperclassmen and you can't talk outside. I'm something of a social butterfly, so having to shut up and do plebe stuff can really bring me down.
My parents didn't bring me up to USMA, they booked my ticket/shuttled to to hotel/shuttled me to USMA on R-Day, so for me, reporting time really didn't matter (It probably wont matter a whole lot for you either). Get there when you get there, do what you're told, and you'll do just fine
This is a pretty big question to ask and not a super easy one to answer. During the AY, most upperclassmen drop the super strict military formalities that you have during training, and a lot of people shift from being seemingly really mean to people who just really care about you/your success.
I wouldn't say that any one pillar is more challenging than the other, but rather balancing the three and keeping sane. In the AY the military stuff tends to come pretty easily (formations/uniforms/reporting to class...) because everyone around you is doing the same thing. The plebe-specific things tend to wear you down a little more, but that's why you do it, to wear you down. Academics have never come easy to me, so I have to work very hard to make the grade, but you quickly realize that you're surrounded by smart people who are (usually) able and willing to help you out.
