1) "… to lower stat" appears to apply based on buffed values (after +X and +X%).
2) The increase is capped so that it doesn't bring the new buffed value above buffed will.
Example:
Let's say I have 81 will, +10 will and +10% will (so buffed will is 100.1, and rounds up to 101, making the buff sqrt(101)=+10.0498 … so probably +10)
And 50 buffed strength (after +X and +X%), so the +10 applies in full - buffed str is now 60.
And 102 reflexes (dtto), so nothing applies (because it's already higher than buffed wil).
And 102 base perception, -5 perception, no % to perception: starting buffed perception is 97 (lower than the 101 buffed will), +10 is 107, but capped to buffed will, so new buffed perception is 101.