My first question is: what are the default settings for Serial1? I'm asking for data bits, parity, stop bits.
My second question: are USB data pins physically connected to RX and TX pins (otherwise, can I use USB connector to Serial1 communication?)?
USB Serial connects to the USB pins. Serial1 uses the UART pins. They are two completely different systems with different pins. Serial1 connects to the TX/RX pins, Serial (CDCACM) connects to the USB socket.