As well as this, not sure if you're aware but pressing F9 during test play brings up a debug window where you can set the values of switches and variables to anything you want, making it quicker and easier to test certain parts. :)
This is because you've overwritten the default update function of your scene but aren't calling the super in it, so all update is doing is whatever you've written. You need to preface your if statement with a call to the base class update function, same as you did with the window.
It's bad form...
If you don't want to go down the route of adding the ability score modifiers as variables in their own right, remember that the mod is just (ability score - 10) / 2 rounded down.
So if you wanted to incorporate, say, the str mod, you'd just do Math.floor((a.atk - 10) / 2)
I do, but my notetag help is free. :P
Now that I think further about it, though, the custom cost display in Skill Core is just a text thing with no eval, so that won't work.
I think the only way to do this is to have a different copy of the skill for each actor that can use it with a Custom...