1. Yes
2. Avoid connecting/disconnecting the Lipo while USB is already connected, connect it before or after the USB is removed.
3. Either LiPo or USB will power the M0, as long as 1 source is there it will keep going, no resets
The onboard charger is intended for USB charging, not solar. If your solar panel can provide at least the required constant current and [5, 6V] then it should be OK.