Jin, I always think that it is a good idea to follow the recommended way in using these tanks.

I use a Jobo and always use both reels, even if one is empty and the full amount of chemistry.

The reasoning is that all development times and liquid flow are calculated with the tank filled to the recommended level and with all the movable parts (reel, film) in. Otherwise the airgap is too much and there is less restriction to the flow when one reel is missing. The increased air space results in bubbles and the missing reel in faster fluid flow (and the chance of streaking).

Recently I moved from hand inversion to a Jobo CPE2+ and still I go for the recommended chem quantity even if only one reel is in the tank (fluid speed/turbulence being here the most important thing)

As for the difference between the FP4 and the Scala pictures, it could very well be problems with the dev gone bad. Why not give it another try with fresh dev and see how it goes.

As for re-using, I do it most of the time, but not up to the manufacturers recommended limit. Only up to the limit that no time increase is required (so maybe 2 or 3 times).