Developer dilution doesn't do much apart from requiring longer development times. Moreover 1+4 gives lower contrast than a stronger dilution. Or did you mean to use dektol for negatives (why??)
Reciprocity failure seems like a very unpractical manner to deliberately manipulate contrast.
Selenium toning if negatives works, but it's really a last resort if somehow you messed up the negative in the first place.
Besides red filters, also consider other colors in particular yellow and orange; they give a bit more control.
I'm still wondering about your preference for graded over multi grade papers?
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