I suspect the Superbe FD is too wide. Its cage is spaced for a 7 speed chain and chainrings.
There's probably a big enough gap to allow the chain to drop off the rings.
You can try to pinch the cage narrower, and replace the spacer between the plates with something a few millimeters shorter. A nylon lift-off for motherboards come to mind. Or a series of small flat washers.
As for chain length, it can still be too long...as you mentioned it seemed to not have enough tension to force the chain to engage the small chainring.
I'd check to see if the pulley cage has enough pull on the chain while you're in the small cog/small chainring combo. You are running a compact crank.
If the chain gets stuck in the space between the big and small chainring, is that still a tension problem in your opinion?
Thanks.