Think of it this way,,,

Would you like it if the ground to the ABS unit in your car was connected to a module, that was grounded through a cable, to the CAN buss and then to the vehicle ground?

Or would you like it better if it was hard grounded at the ABS unit?

Adding that 2nd ground wire on the harness 'helps' if the other ground in said hardness got weak/tarnished, but it does not help if the MPU board has a bad connection through its small power connector.

Do all the grounds, be safe. If you do not do them you may be ok, you may be swapping coils every once in a while. And those old Gottlieb coils cost more every year. One other trick I do is add 2.5a SB fuses in holders at every larger coil under the PF (5 Bank Reset and VUK's love to eat coils).

The coils may be partially baked and internally swollen from being over-held due to driver board issues. Once repaired, if you do not have the mods then do them as they may have contributed.