How to manipulate layer masks in ExtendScript?

To answer #2; making a layer active, you must first know what layer you wish to make active.  The easiest way requires that you know the name of the layer and search for that.  See below:

var myDoc = app.activeDocument; //Alternatively: var myDoc = app.documents.getByName("docName.psd"); to find the document named "docName.psd" if multiple docs are open
var myLayer = myDoc.layers.getByName("targetLayer"); //Finds the layer named targetLayer if it's not in any groups
myDoc.activeLayer = myLayer; //Make the layer the active layer

If your layer is within a group, you'll have to check inside that group, which can be done by changing the second line above to:

var myLayer = myDoc.layerSets.getByName("groupName").layers.getByName("targetLayer");

To find a layer regardless of what group it is in, things get a little complex.  You'll have to, first, find all the groups (photoshop script calls them "layerSets") in the root of the document.  You'll then be able to search those groups for any additional groups and sub-groups within to an infinite extent.

#target photoshop
var myDoc = app.activeDocument;
main();
function main() {
    var groups = [];
    /////////Find Your Base Groups/////////
    for(var z=0;z<myDoc.layers.length;z++) { //Cycle through all the layers in the document
        if(myDoc.layers.layerSets) { //Determine if it is a group or not
            groups.push(myDoc.layers); //Add the layer to the "groups" array if it is, in fact, a group
        }
    }
    ////////Find All Sub-Groups//////////
    for(var z=0;z<groups.length;z++) { //Cycle through all the groups in the "groups" array
        for(var e=0;e<groups.layers.length;e++) { //Cycle through the layers in the current group
            if(groups.layers.layerSets) { //Determine if layer is a group or not
                groups.push(groups.layers); //Add the layer to the "groups" array if it is, in fact, a group
            }
        }
    }
    /////////Look for Your Layer//////////
    var foundLayer = null;
    for(var z in groups) { //Cycle through the list of groups and sub-groups we created earlier
        try {
            foundLayer = groups.layers.getByName("targetLayer"); //Change the "foundLayer" variable to the target layer if it exists in the current group of this loop cycle
            break; //Stop the loop if the layer is found
        } catch(e) {}
    }
    /////////Determine if the layer was found////////
    if(foundLayer != null) {
        myDoc.activeLayer = foundLayer; //Make the layer active if it's found
        //run your script here (recommend placing it in another function and calling the function here instead).
    } else {
        alert("The layer could not be found!"); //Alert if the layer could not be found
    }
}

The downside to the above method is, it can take a long time to run if you have a large number of groups and layers.  There may be a more efficient way to do it than that, but that's the best way I know of.  Anyway, I'm starting to stray beyond the scope of the original question.  I hope this helps you out some at least.  I know it doesn't answer your question in the topic line, but hopefully it answers your other one (and probably then-some).

As far as manipulating layer masks; your best bet IS likely going to be the script listener code.  Simply enable the plug-in, manually do whatever it is you need the script to do; then copy the code for that action to a separate function and name the function based on what it does.  You can  usually tell what part of the scriptListener code is changeable (if any is), so you can easily change that to a variable in order to change it based on what you want it to do/affect if it's called multiple times throughout the script.  Anyway, if you have any other questions, or need me to clarify anything, let me know.

dgolberg