Yes, for the purposes of this challenge, engine horsepower affects TBO to some degree; the effect is more extreme at either end of the horsepower spectrum, the idea being at the high end of the power spectrum that it would bring values more in line with what you'd expect in the real world. In the power range we will be spending most of our effort in future challenges, it will have a less noticeable effect. I may yet revisit that part of the calculator for future rounds of the challenge going forward.
As for the propeller weight dropping to zero at extreme values, I realise two things now - first, I need to specify a minimum diameter in the challenge, and second, we will almost certainly never be working down in that range anyways. The smallest propeller I am aware of in any manned aircraft is 28 inches, on the Colomban Cri-cri, and it's tasked with absorbing some 15 horsepower per engine. I also realise now that in some of my calculations, certain values break the calculations too because I did not put in proper minimum bounds.
Propeller cost and weight is based on a fairly extensive survey of real-world propellers, where it was found that the diameter and the engine power were the biggest drivers of cost and weight - and the equations I generated to simulate these seem break down at sizes below about 40 inches, which is about the smallest widely available propeller size.
I should mention that a very small propeller turning at high speeds will tend towards very poor efficiency in one category or another; they either have very low or very high advance ratios at cruise speed, and may find themselves extremely compromised in a very critical area of performance as a result. Unfortunately it's something I can't easily simulate in the powerplant calculator without making it frighteningly complicated to use, unless I find a way to massively simplify the calculations...
Additionally, any propellers with tip velocities exceeding Mach 0.8 suffer extreme efficiency loss when plugged into the propeller simulation model. As an example, a propeller which has a peak efficiency of 0.86 at its design cruise speed and a tip speed of Mach 0.7 (which are pretty typical values for your average propeller) sees its efficiency drop to roughly 0.80 when turned at Mach 0.75, and it is just below 0.75 when turning at Mach 0.80.
The reason is because of how a an airfoil (which a propeller very much is) works:
As we no doubt learned in school, one of the reasons why an airplane flies (and by extension, how a propeller generates thrust) is that the air flowing over top of an airfoil needs to move faster in relation to the air below it, which according to Bernoulli's Principle results in lower pressure. Now, as the airfoil goes faster and faster through the air, at some point the airflow over the top will start to approach and even exceed supersonic velocities, creating a standing shock wave over the top of the airfoil. This standing shockwave decreases the lift produced by the airfoil, increases drag and in the case of a propeller, dramatically increases noise production - something I think I might try to find a way to simulate for the next round...after all, who would want to fly an aircraft for hours on end with an unnecessarily loud, droning propeller?
The important thing to take away from this is that transition to supersonic airflow begins well before the airfoil's speed itself approaches supersonic speed. Typically, most airfoil shapes (and it is very dependent on shape) experience this transition at a forward speed between Mach 0.65 and Mach 0.8, with the airfoil shape I chose as the default for the propeller simulation, it happens at Mach 0.75.