That film is actually normal because aluminum wears off the impeller and stator which shows as a dark film like that.
FYI, the converter should have been replaced when the trans was built.
You could be right about some of it coming out of the converter though, as when the converter spins centrifugal force throws junk to the outsides and it packs around the impeller blades and sticks in there.
Still some comes loose and you will see it in a new rebuild if running the used converter from a previous trans failure.
As far as kickdown goes, has it worked ok before?
It's pretty tricky to get a kickdown at slightly higher speeds, unless the trans has a part throttle kickdown.
Early trans didn't come with the part throttle kickdown and later ones did.
You could try giving a bit more on the TP by adjusting the linkage and see if it kicks down after that.
PT kickdown valve assembly looks like this.
The add on section is what is the PTKD is if you want to take a look for it.
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