Noctris gave you good advice.
If you have another box, I'd seriously look at installing to it and import the pool. I'd also disconnect my data pool, install a test drive and get FREENAS running smoothly on the hardware. It looks like there is more going on with the gear or the install. So get a stable system and then worry about the pool. Mirrors are pretty safe, and you aren't crashing the kernel on import or anything scary yet. The box is just chucking errors, and there are lots of potential reasons for that.
If one drive or port is the issue. Disconnecting it (try both) may bring the system back online. Degraded is fine... we want access. Then you get the data off there.
Auto importing on a clean install, is also a good test. Just save your existing usb and use a fresh one so you can always go back.
Not too hard to move the pool to a fresh box and try an import either.
You may end up with bad news, but at this point the trick is just to work through it and not panic.