Apply corner radius using setEntirePath()

Here's a roundedRectangle() function with the same syntax as rectangle(), but with an added corner radius argument.

// e.g. of function call
roundedRectangle(
    -25 /*top*/, 25 /*left*/, 50 /*w*/, 50 /*h*/, 10 /*cornerRadius*/);

/*
 * roundedRectangle by Femke Blanco
 * Beta 04/08/2022
 */
function roundedRectangle(top, left, w, h, r) {
    var paths = app.activeDocument.pathItems;
    var path1 = paths.add();
    var p;
    var c = r * ((4 / 3) * Math.tan(Math.PI / 8));  // handle length
    // p2
    point(left + r, top, left + r, top, left + r - c, top);
    // p3
    point(left + w - r, top, left + w - r + c, top, left + w - r, top);
    // p4
    point(left + w , top - r, left + w , top - r, left + w , top - r + c);
    // p5
    point(left + w, top - h + r, left + w, top - h + r - c, 
        left + w, top - h + r);
    // p6
    point(left + w - r, top - h, left + w - r, top - h, 
        left + w - r + c, top - h);
    // p7
    point(left + r, top - h, left + r - c, top - h, left + r, top - h);
    // p8
    point(left, top - h + r, left, top - h + r, left, top - h + r - c);
    // p1
    point(left, top - r, left, top - r + c, left, top - r);
    path1.closed = true;
    function point(x, y, Rx, Ry, Lx, Ly) {
        p = path1.pathPoints.add();
        p.anchor = [x, y];
        p.rightDirection = [Rx, Ry];
        p.leftDirection = [Lx, Ly];
    }
}

check this thread for a similar question

https://community.adobe.com/t5/illustrator-discussions/need-help-rounded-corners/m-p/11182904#M179839

setEntirePath() takes care of the Anchor points, after that you'll have to set the Handles of each Anchor. It's quite a bit of Math, but it can technically be done

Not possible.  You can't make smooth points with setEntirePath().