Paladin Parnell is 3 things: A strong medic, too much like Caira and too much like Slim.
I wanna tackle the first two points here. The main reason why he is strong, is because his weakness, focusing him and somebody else is not super obvious. He also is too much like Caira: Grenades with a 8.8 meter radius that heal 120-125 health.
The 8.8 meter radius is the main issue in this suggestion. I think it makes it too easy for PP to heal more than one target at a time. If you look at the real Parnell, the latest explosion radius I can find about his rockets is 4 m inner radius (full damage) and 7 m outer radius (reduced damage).
To fit PP more in his role of only 1 target at a time, I suggest reducing his radius to 4 meters.
Since I dont want a flat out nerf, I also suggest that his maximum spread of 6 gets reduced to 2, which is what Parnells Rockets have and would make it easier to hit targets with that reduced radius.
I can also imagine how an increased rocket speed would be nice to make him more different from Caira whose grenades travel rather slowly.