How to position a library asset on the document?

placeAsset() behaves in kind of a funny way if there is an active selection. So you need to make sure there isn't one when you place the asset. Then you can just move the asset into place with .move(). You should be able to check its geometricBounds as well (or visibleBounds). Note that in your case it's an array so you may need to do multiple moves.

Here's an example script that assumes the user has selected a bunch of articles. It then goes and places an asset ("T") from the library relative

to each article. Note that it clears the selection first to avoid the above problem.

function main() {
   var
        selectionArray=[],
        i;

// Asset.placeAsset() requires there be no active selection, otherwise
// it gives goofy results. So preserve the selection.
    for (i=0; i<app.selection.length; i++) {
            selectionArray.push(app.selection);
    }
    app.select(null); // Deselect everything
    // Loop and apply placeT() to all selections
    for (i=0; i<selectionArray.length; i++) {
        if(selectionArray.constructor.name==="TextFrame"){
            placeT(selectionArray);
        }
    }
    app.select(selectionArray);
}
function placeT(article){
    var
        leading, p, t, x, y,
        articleBounds=fromBounds(article.visibleBounds);
...
    p = article.paragraphs[2];
...
    // X pos is article's left edge plus its left inset (if any)
    x = articleBounds.x1+article.textFramePreferences.insetSpacing[0];
...
    leading = p.pointSize * p.autoLeading / 100; // xxx not sufficient
    // Y pos is the baseline of the correct graf plus two line heights
    y = p.baseline+(leading*2)/12;
...
    // Place the T
    t=app.libraries.item("Library.indl").
        assets.item("T").placeAsset(app.activeDocument);
    // Move it into place.
    t[0].move([x, y]);
...
}
function fromBounds(gB) {
    return { y1: gB[0], x1: gB[1], y2: gB[2], x2: gB[3],
        height: gB[2]-gB[0], width: gB[3]-gB[1]};
}