No; a USB adaptor is a rather loose term - you can get USB adaptors to connect to networks (both wireless and wired); it means something that adapts USB so that it can be used for something else - I...
RS-232 serial ports are far slower than USB ports. With very few exceptions, devices that use USB ports would be impractical to use on a serial port.
If it is an iPod Touch, yes, you can work on an iPod or use the iPod while it is charging.
Yes, I do it all the time.
NO