1. In this case, using OnDrawTooltip is very expensive.
This action will be called every frame, for each visible marker.
Subscribe to OnlineMaps.OnMapUpdated, and update the tooltips only when the map is updated.
Something like:

private void Start() {
...
    map.OnMapUpdated += OnMapUpdated;
...
}

private void OnMapUpdated()
{
   foreach (OnlineMapsMarker marker in map.markers) DrawToolTipForMarker(marker);
}

private void DrawToolTipForMarker(OnlineMapsMarker marker)
{
    TooltipData tdata = marker.customData as TooltipData;
    if (marker.inMapView)
    {
        if (tdata == null)
        {
            // Create tooltip
        }
        // Update tooltip
    }
    else if (tdata != null)
    {
        // Destroy tooltip
    }
}

public class TooltipData 
{
    public GameObject instance;
    public UIWidget widget;
    ...
    // Any other data you need
}

2. You can store all the tooltip data in marker.customData.
3. If the marker is not on the screen, you need to destroy the tooltip.

The built-in tooltips are drawn using the old GUI system.
If you want to draw this with uGUI, you can change this.
Examples:
http://infinity-code.com/atlas/online-m … ample.html
http://forum.infinity-code.com/viewtopi … 1615#p1615

To always show tooltips use «Online Maps / Advanced / Show Marker Tooltip - Always».
To change the location of the tooltips, use OnlineMapsMarkerBase.OnMarkerDrawTooltip.
Example:
http://infinity-code.com/atlas/online-m … ample.html
Using this action, you can make almost any behavior of tooltips.