Determining maximum available braking torque on DC Drive

In answer to your questions:
Load on motor should be constant throughout entire decel.

If I lengthen ramp time down, with (braking) torque around (or below) 40% of drive max - then I get constant decel rate all the way down. It is only when I go above this level that I see two separate decel rates. Initial rate corresponds to what I am aiming for (say based on 75% of max torque), and then it switches (at some lower RPM) to lower decel rate.

I know that aiming for constant decel rate via torque control (rather than speed control) might seem a bit backwards - but that is way that initial app was set up (I assume because torque control can control more quickly than speed control. Maybe things are different on newer drives...)

And, I am not sure what you mean that I am not commanding a torque, but only changing limits. When I tell the drive to give me 50% (of max.) torque - shouldn't the drive be giving me that torque (whether it is getting power from regen system or from grid) one way or another?

When you command both speed and torque, they are basically both just limits.

Whichever limit is hit by the load is the controlling factor.

So, for example, if you command 50% speed and 50% torque, the load may cause the motor/drive to hit the torque limit before it can reach the speed limit, so it might be running 40% speed at 50% torque...

On the other hand, if the load is light, it might hit 50% speed and only be putting out 25% torque to do so.

But... if I am operating in torque control mode (I am speaking now about ACS800 drive) - I understood that it is ignoring speed (of course, where I am not exceeding limits) - and just controlling trying to achieve a target torque (which will in turn of course affect speed). If this torque is negative (as it is in my case) - this will reduce speed.

Yes, that should be the case...

So, if you are going forward at 50% speed, and command torque to -100%, the load should decel to 0 and beyond to -100% speed, if you don't change any commands... (and the load is less than -100% torque)

Yes - that is exactly what I expect should happen if I keep torque at constant negative value. But... when I reach zero speed (end of decal ramp), at that point I switch back to speed control, with speed = 0.

As few things
I just downloaded the service manual for that drive it looks like a nice unit with many options
Some modes are even regenerative. I was surprised however that for a new digital design drive they are still only using only 3 SCR’s on the basic unit.
But from what you describe and from the posts there is no DB on this drive. For a DB you must have an armature contactor to disconnect the power converter from the motor armature and connect the DB resistor so we can stop talking about the DB. And I really don’t think it’s a regenerative drive, the model number would tell us a -2 would be non regen and a -4 would be regen.
So what you are trying to do is control the motor speed by controlling the motor torque. And it seems like you are doing that in the run mode.
But when you want to stop the motor you want to ramp down the speed using torque control.
That’s actual very simple just lower the torque command / limit the motor will come to a nice complete stop. You do not need to change to speed control at all. If the torque limit is lower than what is needed to move the load it will stop. Keep in mind you only have a very small torque window to control the motor speed. If the torque limit is to low the motor will stop and if it is to high the motor will runaway and go to full speed. If while it’s running your torque requirements change up or down the motor will either stall or runaway that’s why you have the speed feedback in the torque control loop to control the toque to get the desired speed. Your term reverse torque I think is confusing things that would normally be used if you want to reverse the direction of the motor in this case you are not reversing the direction. Just remember in torque if you want to stop the motor just set the torque limit to 0 the motor will stop and if you want it to move just increase the torque limit until the motor is running at the desired speed. Trying to switch between torque control and speed control is causing you problems. The drive system will switch from torque control to speed control automatically when the torque limit is larger than the torque required to move the motor. That’s why I recommend that always set the speed limit to just above the desired operational speed. So it that happens you don’t get the large speed jump
One thing to remember the torque limit just limits the amount of current available to the motor it cannot force current into the motor. The motor draws what it needs to do the work. The way the drive controls the current available is exactly the same way it controls the voltage and that’s by controlling the phase angle of the SCR gate. The end result is to control the voltage to motor armature.
One other thing the Acceleration and deceleration ramps only apply to the speed reference change not the torque reference. Example a ramp up 10 sec is from 0 to 100% speed command and the ramp down is from 100% to 0 speed if your speed reference is already set a 50% and you change it to 100% it will only take 5 sec to reach the new set point. In torque control the reference ramp must be controlled from outside the drive controls. A change in torque reference would only take less than 8 ms to change in the drive.
One other thing when dealing with DC motors you cannot run them at very low speed and high torque / amps for very long you will burn up the commutateor and never stall them at high current. I have seen it a few times.