Hi, I have some direct experience with long duration stratospheric superpressure balloon design and operation. I'd like to highlight some some important concepts long duration balloon issued that are often non-obvious. I realize these things are also probably non-trivial to implement, but I'll throw them out here since you do seem to be making an significant effort to be accurate to real world physics. My apologies if some of this is review for some of you.
Zero pressure balloons (non-elastic film) inflate as they climb until they reach a fixed maximum inflation volume, and then gas overflows out a hole at the bottom or a simple overpressure valve.
Superpressure balloons (non-elastic film) inflate as they climb until they reach a fixed maximum inflation volume. The gas cannot escape, so as they continue to climb up into lower pressure air, the absolute pressure in the envelope remains constant, while the outside air pressure falls. This pressurization, without volume increase, causes the balloon envelope to maintain a constant density. It will stop climbing when the air density outside equals the balloon's density, or the tension in the balloon pressure vessel skin exceeds the strength of the envelope material. Superpressure balloons are a pretty ideal place to apply the ideal gas law.
In the real world, temperature will go up and down every day, and moles of lift gas will always go down every day, in both ZP and SP balloon envelopes. ZP envelopes are expected to collapse when they cool at sunset and become negatively bouyant. Mass must be jettisoned at that time on a ZP, or in a few hours the balloon will gently arrive at the ground/ocean surface. This raises the neutral bouyancy float altitude slightly. At sunrise, the ZP heats up causing gas to overflow. This also raises the neutral bouyancy float altitude slightly. Repeat until you are out of ballast. Usually the last sunset after running out of ballast marks the beginning of an gentle, but terminal, descent. However, if you are descending so slowly that you are still in the air at sunrise, the sun will heat the gas for one last climb up to float altitude. After an uncontrolled night landing, if the balloon and payload remain undamaged and not caught on anything on the ground, the balloon will take off again for another day of flying. These bonus days often do not make it back to full float height, and often sink to the ground after noon, but before sunset.
Superpressure balloons ALL WILL LEAK at a varying rate that is governed by the fixed leak hole size, and the fluctuating pressure differential. There is no such thing as a leak-tight balloon when you're considering flights of up to one year in duration. The leak's effect on a balloon can be simply described by the ideal gas law - you lose gas moles permanently through a leak. That means if temperature were to remain constant, your internal absolute (and differential) gas pressure is going to decrease over time based on how many moles remain in the envelope. What makes this maddening to measure, in reality, is that temperature changes significantly every day, which changes the internal absolute (and differential) pressure, masking the pressure drop from losing gas molecules.
To further compound the headache- the lower your differential pressure (inside envelope lift gas to outside air), the slower you lose moles of lift gas out the leak.
To usefully quantify the "leakage rate" for a particular balloon, you need to use a a special set of units relating (pressure * volume / time). Commonly used units are (Pa m^3 / s ) or (atm cc / s) for example.
This "leakage rate" number will stay the same no matter what the pressure is in the balloon or how many moles are lost.
When you lose enough moles of gas that on some fateful day your differential pressure drops to 0, the superpressure balloon becomes a zero-pressure balloon, and will die in the same manner at sunset as a ZP balloon does.
Thus the SPB envelope must be designed to handle the differential pressure swings cause by daily/seasonal thermal radiation changes, as well as the average continual pressure drop due to the continual loss of lift gas. This is a difficult challenge given the properties of materials available today, with very little margins.
I'll wrap this up here for now. I am also willing to contribute to coding of this mod if i can be if help, as I'd like to see these effects, and others not mentioned yet, eventually modelled in the KSP balloons. (I suspect that having an envelope visually collapse like a real ZP may not be possible in this game engine though)