does changing UART stop bit number configuration (receiver side) affect message correctness
The stop-bit has a logical high level, it's the last bit that gets transfered if you transmit a byte over RS232. In case that you don't do any transfers the line is idle, and this is signaled as a logical high level as well.

If you increase the number of stop-bits on the sender side of a RS232 connection the receiver side will interpret the first stop bit as a stop bit, and all following stop bits as idle time on the line.

Effectively for each additional stop-bit you just add a small delay between the bytes transfered over the link.