What I like to do with drives is to wire the start contacts through one of the drive's normally closed relay outputs, and then control that relay through Modbus. Activating the relay turns the drive off, deactivating turns the drive on. That way, I can have the drive default to a running condition if the communication is lost, and on the ABB drives you can have the reference follow Constant Speed 7 upon a loss of communication. You can then set Constant Speed 7 to whatever value you want for a fallback.
This might also help to solve the E-stop problem.