Today a quick update on the project. The USB to serial adapter has been replaced by a HC-05 Bluetooth module. The test setup itself did not change other than my table became a lot cleaner as less cables are needed. The computer sends characters defining the channel and speed serially over Bluetooth to the test setup. All the motors immediately reacted to the computers instructions. Another milestone achieved in this project!
The only problem I had was to find the correct COM port om my computer, not something to worry about as a mobile phone will take over the transmitter part.