I think it is cheaper by coach. plus, If you have a tech stop at MAN, you may as well run the service from MAN.
For the tech stop at MAN from LBA, you would need:
2x airport fees
extra fuel to hop to MAN, then refuel at MAN for the trip
could affect crewing hours
would need to be guaranteed a slot at MAN for the tech stop
With a large ops programme from MAN, id think it unlikely they would use DSA for a tech stop.