Gee, I'm getting into territory where Mr Planck will give me a few deserved slaps for getting it wrong but, you have immediate issues.
you can't define the *same* units[]= in two separate pbo's.
the effect of nullifying EVENTS is ignored in the required pbo (it uses it's own definition)
I assume? your mission sqm addons[]= points only to your addon and shouldn't make any reference to the original.
I fail to see the purpose of referencing the orginal at all!!!
There is an error in the original specifically SNOW_ENVIRON_SOUND should not have a semicolon at end of it and it buggers the context when it is used.
as far as I can see, what I would do, is:
change your mission.sqm to refer *only* to your patched version. Make a complete copy of the original and only alter the #define EVENTS as you have done.
make no reference to the original, in your new patched version.
do not have BOTH pbo's accessible to the ofp engine during mission design. It is garanteed to get the wrong one via murphy's law.