Help editing the ease() velocity?

The expression bit throws me, but this ease expression will allow you to "range map" from the sliders to your scales:

var easedScale = ease(effect("Slider Control")("Slider"),0,100,25,75);
[easedScale,easedScale]

the 0,100 represents the minimum and maximum values in the slider and the 25,75 sets the min and max scale values.

The speed graph can help with creating a pinch, but it does sound like you want to control this through the expression and I take it you can't apply the speed graph to the slider keyframes.  If so, it sounds like you need an IF statement, so that:

If (slider < 50) {
ease up
} else {
// when slider is beyond 50
ease out
}

Without seeing what you have so far, that's the best I can do.

It is really easy to get lost in the weeds when it comes to expressions and you end up spending more time than you should just trying to program something.  One alternative to consider is to use Pre-Comps with Responsive Design - Time enabled.  This lets you use keyframes to get the perfect animations, but then you can extend them, have them pause and all sorts and you never need to go near an expression.