Not sure about your last point but in serial comms there are start and stop bits and sometimes parity. We generally used 8 data bits with no parity so in effect there are 10 bits per character including the stop and start bits. That pretty much matched up with file transfer speeds achieved using one of the good protocols that used sliding windows to remove latency. To calculate expected speed just divide baud by 10 to covert from bits per second to characters per second then there is a little efficiency loss due to protocol overhead. This is direct without modems once you introduce those the speed could be variable.