...and the ridiculousness continues

Not that I care about mpg, I drive a jeep, but when decelerating in gear there is no fuel being used? Vs just coasting in neutral

Yes. That's correct. Coasting in neutral == fuel burned to keep it idling. Decelerating in gear == fuel cut. Basically all OBD2+ era cars operate this way, 20 years now. That's one reason modern cars don't "crackle" or backfire a little bit on deceleration like old carbureted cars and early EFI/MFI cars did... there's no fuel being injected on decel.

But once you go a constant speed or get close to where it idles it starts feeding full again?

Are you talking about a manual or automatic? If manual, it will continue fuel cut as long as your foot is off of the accelerator. IDK what exactly happens when you let it decelerate enough to stall without giving it any additional gas. I suspect once the rpms go below idle rpm it probably will begin to add fuel again in hopes to not die but if you don't give it gas at that point it's going to stall no matter what.

That's hardly the normal use case. Most people will not let it get down to 600 rpm decelerating in gear without downshifting.

The manual. I know it's gotta start putting fuel back in sometime I just didn't know when.. In slow traffic in 1st and some times 2nd gear I'll slow down and without pressing the gas it'll still go even on a slight incline.