I'm not sure about the Kindle Fire but I do know that Android tablets can integrate with Calibre with Calibre Companion. I used Calibre Companion with my Nexus 7. If you get a bigger Android tablet (9 or 10 inches), reading PDFs should be fine. I could read PDFs on my Nexus 7 but it wasn't an optimal experience. There are apps in the Play store that read mobi and PDFs.
Pretty much all the modern Android tablets have integrated speakers and headphone jacks. I thought my Nexus 7 had great integrated speakers that produced good sound. The Play Store has a wide variety of apps including education one. It also comes with a stock word processor that is very basic. The one complaint about Android is that a lot of apps aren't tablet optimized. The great thing about Android tablets is that some of them do have expandable storage. Samsung is really great about including expandable storage on their devices. The LG G Pad also has expandable storage. All of the Android tablets I've encountered have front facing and back cameras. Some, like the older Nexus 7 (2012), only have a front facing camera. The newer Nexus 7 has a back camera though. HTH.
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