Corossion on the plugs is not very common, but possible.
Depends on the enviroment you're living.
I personally had a probem caused by condensation from the AC that dropped on the TCM.

Did they check then correct temperature of the ATF oil while measuring the level?
This is extremly important.

I don't understand why they didn't get any P codes.
Did they test with a DRBIII tool, or just with a ODBII tool.
You will need a DRBIII tool, to get the codes from the transmission.

It's also possible that the valves do not work correctly.
The first i thing i would do is a complete atf oil change (that means also changing the oil in the lines, atf-cooler...) including flushing the system and change the filter.
Maybe there is some dirt in there which causes a little clogging.