Mostly did it that way because i wanted to keep the settings screen somewhat simpler, because otherwise full control would have about 16+ sliders for the diminishing return curve lol.

The long and short of it is the original mod had a "postprocesscurve" this is basically a "if you have X value make it Y', so if i gave it a 3 i could make the curve turn that into a 2 for example. So i basically tried my best to map a single number to an entire curve because the curve generally is to help so low manipulation increases still feel like they do something, while higher manipulation doesn't instantly break the game (i'm not any sort of math genius so there's probably a better formula.)

tldr; the current defaults should be fine for a more-vanilla-aligned playtrough; otherwise on my own personal run i set the base 2 sliders to .25, while i set the diminishing return sliders to 25%

I kinda put 75% as a default as it was in that "felt good" territory when i was playing around with the settings. As stated in the mod, 100% basically is as if the diminishing return setting was off, 0% is gonna be the most aggressive. tldr; if you set it to 0% and crank the base value of 10x your manipulation value you probably still won't hit the absolute floor of cooldown.

What does it represent? Eh, the long and sort its effectively just a harshness of a curve. If you want the exact math, go to the github link, check out the "rebuildCurve" method for a direct look.

Is the 75% diminishing returns setting the default? What does it represent?

If your mod isn't properly updating to the new version just ubsub/resub. If you see the mod verison is 1.1 then its the correct one.
For some reason steam is just not wanting to update for some reason.

ok update done, diminishing return slider added.

I'm working on a diminishing return setting right now. Ii'm hoping i can have it out soon, but trying to get the curve to feel reasonable on a 0-100% scale.