I gave it a try - as usual, there are quite a few tricky things to get right!
thanks to rokstars322, I was able to edit your sample by deleting one of the effects from the Created_Objects family - since I don't have a C3 license yet either. All the code is there, so you can just add the Contrast effect back in to make it work...
your disabled code was not working because you were checking for a click on Created_Objects, Selecting the object, then the next event was undoing it (because the mouse click was still true, and now there was a selected object to unselect).
so I re-structured that whole section... (I also deleted the fullscreen code because it was running every tick and making my screen flicker)
Having a Control Panel adds a whole new level of complexity and new problems to the logic of an object family. If the panel is over an object, any click on the panel may also register as a click on any objects below. In your case, that would cause your code to mysteriously change selected objects without you knowing why. AND if there isn't an object under the panel, then the code would think you are trying to un-select objects! (which would also cause the panel to disappear!)
the solution I have used to fix these issues in the past is to have an object be part of the family and use it as a shield. So, I made a Panel_Shield sprite, set it to be invisible, and made it the same size as the panel, and pinned it to the panel. When ever the panel is brought on the screen, the panel_shield must be moved to the top of the layer because if there are any objects above it, you will have trouble. (or keep it on the UI layer - I had to delete layers because of free version limitations).
the panel_shield is given a unique "ObjectType" so that we know to ignore clicks on it. It is also important to always use "Pick Top Instance" when doing any mouse button events so that the shield properly blocks clicks from objects we didn't mean to click.
rokstars322 showed updating the Created_Objects when the slider "On Changed" trigger fired. that only updates the value when the slider is released. So, I followed your code where the value is updated every tick so the user can instantly see the effect.
and, like Dop said, you need to store the values in instance variables in the family...
https://www.rieperts.com/games/forum/effects.c3p