ukpbz,
You can tell which aircraft is selected by the small black triangle in the left margin. If you don't want the aircraft de-selected then keep your cursor away from the map as hovering over an aircraft on the map selects it. As for the altitude sort changing - no it does not. I sorted my aircraft, both live and network, by altitude at least ten minutes ago and they are still sorted highest to lowest, top to bottom. Once again, keep the cursor away from the map if you don't want things to change in the table.
Tom