In your case, I tested it because you wrote that elevations is not working correctly, and found that the wrong parameters are used.
You only need to replace one line in this method to make it work.

...
OnlineMaps.instance.GetCorners(out tlx, out tly, out brx, out bry);
...
float el = OnlineMapsElevationManagerBase.GetUnscaledElevation(longitude, latitude, tlx, tly, brx, bry);
...

I've read your post here
http://forum.infinity-code.com/viewtopic.php?id=1056
and the above code should work.

I should place my Gameobjects on top of the terrain, just like 3dMarker, based on their Longitude / Latitude.
I gave a look to the 3dMarker function to do this, but I could'nt understand the right way...

GetUnscaledElevation requires a point in the scene relative to the top left corner of the map.
The right way:

float el = OnlineMapsElevationManagerBase.GetUnscaledElevation(p.x, p.z, tlx, tly, brx, bry);

To do this, I'm using your suggested code:

public static Vector3 GetWorldPositionWithAltitude(double longitude, double latitude, float elevation)
    {
        double tlx, tly, brx, bry;
        OnlineMaps.instance.GetCorners(out tlx, out tly, out brx, out bry);

        Vector3 p = OnlineMapsTileSetControl.instance.GetWorldPositionWithElevation(longitude, latitude, tlx, tly, brx, bry);
        float el = OnlineMapsElevationManagerBase.GetUnscaledElevation(longitude, latitude, tlx, tly, brx, bry);

        float yOff = elevation - el;
        yOff *= OnlineMapsElevationManagerBase.GetBestElevationYScale(tlx, tly, brx, bry);
        yOff *= OnlineMaps.instance.transform.localScale.x;
        p.y += yOff;

        return p;
    }

Unfortunately the Gameobjects seem to be placed at a wrong height: someone is more or less right (on the surface), but most of them are a lot under or over the surface.
I've tried with Mapbox, ArcGis and Bing elevation, but with all of them I get the same result.

I'd be glad for your help!
Thanks!