>> Matthew:

“[...] Mobile smartphones, music players, etc. are not able to mount. (Confusingly, my Android phone is accessible through ADB.) [...]”

WRT this point, those phones and music players are probably not offering a Mass Storage interface, but an MTP interface, which requires specific software supporting that protocol to be used with them.

As for your dmesg output, I've only seen that in cases where either the USB port/controller or the USB device/cable were broken. Not saying yours are.

>> Matthew:

“[...] I should also add that these devices work fine on other distros, so they aren't failing. [...]”

Other distros on that same machine?