So here's the follow-up:

IT'S NOT POSSIBLE.

I've spent at least 5-7 hours on the phone with 365 support at this point, and all we were doing was figuring out workarounds. There's certainly no straightforward solution.

This is infuriating. The fact that MS has neglected this is beyond reason. The ironic part to me is that within Outlook, this works really, really well. In the Exchange Admin panel I can create a shared mailbox and assign fine-grained permissions. Then each user can easily use File>Open>Other User's folder and enter the name of the shared mailbox and the type of folder they want to add. Then it can be manipulated just like any other calendar in Outlook. It works really well.

But you want to see that calendar on a mobile device? Sorry. Not going to happen.

I did find an extremely awkward workaround to view a shared calendar, but you had to go into the OWA portal, add the shared mailbox as a favorite in Mail, then add it as a calendar in the Calendar page, then download the [absolutely horrible] OWA app for iPhone or Android, enter your password into that app twice for some reason, and then you can view the shared calendar, but only in that OWA app and only separately from the others by swiping from the left and selecting it manually. Contacts from shared mailboxes do not show up at all on mobile in any form whatsoever. This wasn't an acceptable solution.