DO NOT use a different GDFS than your own. It WILL BRICK your phone.
If you have already written a different gdfs onto your phone, than your phone is no good other than being used for spare parts.

I don't know why the phone won't connect. Try using different programs. You can even try the davinci client just to see if the phone is discovered.
I don't know anything about service cables, but you could try them out. I erad in your post that you were thinking about buying a cruiser cable. I don't know about this, but perhaps it would work.