Is it safe to connect the individual lines of a split USB cable to each individual port on the arduino? I.e tx, rx, 5V and GND. If so should the 5V be connected to the 5V pin?
You should take view into the schematic of the Arduino Mega.
USB cables don't have Rx or Tx.
The USB signal names are D+ and D-, and as has already been said they are not Rx and Tx.
You can't do what you think you want to do.
Okey, thanks for the replies. I will use a dedicated USB to rs232 adapter + external power instead.