The 14km ceiling is courtesy of the Juno engines. They lose their power at higher altitudes. In practice, during my flight to the arctic (One way, mind you, wouldn't have enough fuel to get back), I've never surpassed 10km. It was just a painfully long cruise at 300 m/s.
Another thing, I have a hint for you: In editors, try to pitch the wings up by about one tick of the angle snap placement, and pitch the stabiliser down one tick (or two ticks if you use the symmetric plane tail, or more if your plane is underpowered, though that will give you more air resistance). That way, the SAS will have an easier time maintaining pitch. You can then go AFK for about 3-5 minutes at a time and just correct the flight when you come back.
For the rudder, you can use the the tail fin or standard canard - those parts can have things attached to them. And for the stabiliser, I've had best results with AV-R8 winglet (or another standard canard). These have fixed bases, so it doesn't look too silly when they move. Although you could probably use anything, as long as it generates lift.
For the aircraft I've posted earlier, the T-tail is a necessity. With a standard tail configuration, those engines would give no thrust for some reason...