What you said in your post makes sense but as a JW I always understood that "Jehovah's underserved kindness" had to do with how he made a way out for mankind by sending his Son Jesus to die for our sins, even though he wasn't obligated to.
I never once got the idea that they said that we weren't worthy of Gods kindness only that considering how Adam and Eve went against the arrangement he set up for them, he didn't owe mankind on the whole, a way out of the sin and death we inherited from them. On the contrary, they indicated that we were entirely eligible for his love and encouraged us to pray to him for guidance, help. "Throw your burden on Jehovah...he himself will you sustain" was a major theme in Kingdom songs etc.
To be fair, as far as the endless treadmill of service, what I remember is that spreading his message to others was a way for us to show our appreciation to him for the hope he provided to mankind rather than as a way to earn his love. They indicated that yes it was impossible for us do enough to deserve everlasting life. Instead, we needed to show obedience to him to show our appreciation for the undeserved "way out" he provided to mankind through Jesus. But therein lies the problem...obeying Jehovah meant obeying them and quantifying to them on a time sheet each month, how much we personally appreciated Gods underserved kindness to us.
Many of us ran into problems when the lines became blurred and we lost sight of what we were doing and why we were doin it and who we were doing it for. That is the problem when you are following an organization that has a factory and has a product to put out and is concerned with numbers.