So, I just thought of something, since Heart is red in color, is used for longer period of time and is basically a muscle, wouldn't it be Slow Twitching Muscles and hence being a target to the… gelling. So, people who ate devil fruit when submersed in the water, according to your theory, should... kinda die.
Wow, I kinda had an, "…oh shit" moment when I read that xD. Alright well theres different stages of gelling. Being a cook I'm pretty good with consistency and fluid gels and what not. The muscles wouldn't completely cease moving, but thickening would slow them down substantially and lower the heart rate to an almost hibernating state. So they wouldn't die, just their bodies would lose the ability to create enough energy to move, but enough energy to function. Until their lungs filled with water and they drown of course.
This brings up a good question of what a fishes heart functions though. I'll have to look into this.