If a transaction is not broadcasted yet, does this also check if the inputs are signed or not?
Yes. It does every check that would occur if the transaction were broadcast. It is the exact same code paths as the actual mempool acceptance logic, minus the part where it is added to the mempool.
Or, what are all the "reject-reason"-s messages I can receive? Couldn't find it anywhere.
All possible validation results can be found in the
TxValidationResult enum at
https://github.com/bitcoin/bitcoin/blob/master/src/consensus/validation.h#L24Does it test against default bitcoin core configuration or your own bitcoin core configuration?
It tests against whatever is currently running.