Mike, your old camera is doing quite well. Once I worked out the various lenses and how they added to my possibilities I was off and away.
The camera itself surprises me with some of the things it can do, quite a neat package better than I thought.
I especially like the 400T and have taken it out a few times to understand what does and does not work.
So far, there are no negatives:-))
I can sympathise with the dark slide mix up, you really are caught between the Devil and the Deep Blue Sea. I suppose you could use the clear but processed film, as tests for evenness of developing?