I removed the plant parts from the water and glycerin solution after 4 hours.
The next morning after they dried, I noticed they were definitely more pliable.
Not easy taking a photo with an iPad of your own hand while holding something.
I removed a test from the denatured alcohol and glycerin solution after 16 hours. It was inflexible and snapped.
I suspect the alcohol solution would be better for preserving the plant parts, rather than for my purposes. The jar does not take up much space so I will check them daily for improvement.