I'll let you know what I understand of this.
Yes, you will be getting separate MBs--one for being a passholder, and ones for each of your hotel stays (unless they change this at some point).
Regardless of how many MBs you receive, what matters is which ones you have associated as 'active'. You basically manage all of your MBs in your MDE account and there you tell it which ones you are actively using. Again, the MB itself does not contain your info in it, it merely triggers back to whatever is reserved/Fastpassed, and whatever other things are in your MDE. So yes, you could get your AP Magicband and use only that one regardless of how many others you get.
For your passholder MB, you should log into your MDE and look under your Magicbands link and you should be able to personalize and complete the band for shipping (which will be mid-March).
As for the April reservation, I thought they were sending bands out 30 days prior and that you had up until a certain day to customize. I believe if you have missed that deadline then you receive the generic gray band when you check in.
Again, I'm sure others will have more detailed (or corrected info), but this is what I know of it myself.
Good luck and enjoy your trip!