How to delete the original object and switch to millimeters as the unit of measurement?

Hi everyone.

I accidentally found a script that works well with the canvas.
However, I noticed that the units are in pixels, and I want to change them to millimeters, but I haven’t been able to do so after multiple attempts.

I also noticed that after running the script, the old objects remain in their original positions. I tried using `sel[j].remove();` to delete them, but I haven’t been able to get it to work no matter where I tried.

Could you please take a look at this?
 

//#target Illustrator

//  script.name = fitObjectToArtboardBounds_v2.jsx;
//  script.description = resizes selected object(s) to fit to the Artboard(s) Bounds;
//  script.required = select something before running; CS4 & CS5 Only.
//  script.parent = carlos canto // 08/05/12;
//  script.elegant = false;


// script.updates = first version fitted a single object to the active artboard.
//                                      for this version, I added options for fitting selected object to all the artboards.
//                                       it also works with multiple selections, all selected objects will be processed at once.
//                                       Bonus: there's an option for adding padding to object's final size.


var doc = app.activeDocument;
sel = doc.selection;
if (sel.length > 0) {

    var title = "Suitable candidates";
    var msg = "Enter the required values\n";
    msg += "Positive numbers: outside the canvas; negative numbers: inside the canvas. Units: px/pt";
    msg += "\nEnter 0 to use the drawing board's boundaries";

    var margins = Number(Window.prompt(msg, 0, title));

    var allArtboards = Window.confirm("Yes - All Artboards \nNo - Current Artboard", false, title);

    for (j = 0; j < sel.length; j++) {
        // get selection bounds
        var boundsdiff = getPgItemGBoundsvsVBoundsDifference(sel[j]);

        if (allArtboards) {
            //alert(allArtboards);

            var ABs = doc.artboards;
            for (i = 0; i < ABs.length; i++) {
                var activeAB = ABs[i];
                var ABprops = getABbounds(activeAB, margins); // get top, left, width & height

                var sel = sel[j].duplicate(); // doc.selection[0];
                fitPgItem(sel, ABprops, boundsdiff);
            }
        }
        else {
            //alert(allArtboards);
            var activeAB = doc.artboards[doc.artboards.getActiveArtboardIndex()]; // get active AB
            var ABprops = getABbounds(activeAB, margins); // get top, left, width & height
            var sel = sel[j].duplicate(); // doc.selection[0];
            fitPgItem(sel, ABprops, boundsdiff);

        }


    }
}
else {
    alert("select something before running");
}


//--------------------------------------------------------------------------------------------------------
// move and resize pgItem
function fitPgItem(pgItem, containerProps, pgItemBoundsDiff) {
    var sel = pgItem;
    var ABprops = containerProps;
    var boundsdiff = pgItemBoundsDiff;

    sel.width = ABprops.width - boundsdiff.deltaX; // width is Geometric width, so we need to make it smaller...to accomodate the visible portion.
    sel.height = ABprops.height - boundsdiff.deltaY;
    sel.top = ABprops.top; // Top is actually Visible top
    sel.left = ABprops.left; // dito for Left

    sel.selected = false;
}
//--------------------------------------------------------------------------------------------------------

//--------------------------------------------------------------------------------------------------------
//returns a pageItem's VisibleBounds vs GeometricBounds Difference
// needed to substract from an item's width/height since they're based on Geometric width/height
function getPgItemGBoundsvsVBoundsDifference(pgitem) {
    // get selection bounds
    var sel = pgitem;
    var selVB = sel.visibleBounds;
    var selVw = selVB[2] - selVB[0];
    var selVh = selVB[1] - selVB[3];

    var selGB = sel.geometricBounds;
    var selGw = selGB[2] - selGB[0];
    var selGh = selGB[1] - selGB[3];

    // get the difference between Visible & Geometric Bounds
    var deltaX = selVw - selGw;
    var deltaY = selVh - selGh;

    return deltaxy = { deltaX: deltaX, deltaY: deltaY };
}
//--------------------------------------------------------------------------------------------------------

//--------------------------------------------------------------------------------------------------------
// returns an Artboard with top, left, width & height properties, including Margins
function getABbounds(artboard, padding) {
    var activeAB = artboard;
    var margins = padding;

    var abBounds = activeAB.artboardRect; // get bounds [left, top, right, bottom]
    var ableft = abBounds[0] - margins;
    var abtop = abBounds[1] + margins;
    var abright = abBounds[2] + margins;
    var abbottom = abBounds[3] - margins;

    var abwidth = abright - ableft;
    var abheight = abtop - abbottom;
    return AB = { left: ableft, top: abtop, width: abwidth, height: abheight };
}