Did a bunch of different tests, and everything seems to be working correctly and consistently across a bunch of different configurations.
Excellent work! :D
Mechanically everything seems pretty sound, however there is design issue for player clarity.
Representing a single flat growth parameter with the current numbers works perfectly fine and makes sense to the player.
However, When using percentages it can be confusing to the player, because at low numbers you can add quite a few points without visual changes to the number before the integer rolls over (it does store and rolls over properly if adding later, just visually confusing).
So, an option to display the total augmenting percentage instead of the stat itself would be a good option on the single stat percentage setup.
Also, design wise, Multiple parameters represented as a single number need a different number displayed.
The most logical number to the player would be the total times the plus button (subtracting minus button count of course) has been added and confirmed to the stat (I say plus button because point cost can be different for different stats).
This gives a clean count of how many times they have added to the stat, and the underlying functions are no different than they currently are.
Just making more sense to the player.
It would also be good to give this option on single stat configuration as well, as it can give nice alternative setup if the developer likes that representation better.