marhsava2

Yes, the only reason why there are earth on output terminals is for convenience. Surfix cables, example, are now terminated  neatly,  without the need to extend the earth core to a case stud. 
But in the end that case earth and terminal earth are one and the same thing. On the Axpert both the in and out earth terminals are wired onto a single internal case stud. 

I know it's strange, but the AC Earth in and case are tied together anyways. I guess they simply cut costs of another terminal, as that would be tied together also.
Same as on my MUST inverter.

yes, you can do.

Either will be safe with one panel
15 amp DC breaker and or fuse
Closer to the inverter/charge controller
Always earth your panels preferably to your common system earth point.