Mr Optimistic;
Still pondering effect of water on properties of air; aero and thermodynamically. Don't wait up expecting an answer anytime soon.
It is always a hazardous enterprise when a pilot, untrained in the field, enters the arena of the engineer but that doesn't stop questions!
The question of droplet break-up is, (I sense) different than the question of how water behaves at, and more importantly affects the boundary layer.
I recognize that answers aren't easy or perhaps even available (as per your caution!), but the question in the paper that is being addressed is the character of droplet breakup and I believe many are curious how that relates to effects upon airflow, (aerodynamics), specifically, (the question being considered here is), does heavy rain affect the airfoil stall speed and if so how? As mentioned in the thread by someone, wing-loading at a critical time is going to change that behaviour which may be the reason we don't see the material effects, (stalls, etc), of heavy rain very often.