In the meantime, this awesome community ported LineageOS 14.1 based on Android 7 to the FP2, and many prefer that already compared to Fairphone Open OS.
Adoptable storage is a feature of Android 6, so yes, that works, you can setup your Micro SD card in the phone in that mode.
I don’t know about Micro USB sticks, but I could read normal USB sticks via a Micro USB OTG adapter just fine even back in the days with Android 5.