No, if you were praying in the direction of the moon for example, you'd be praying towards it's 2 dimensional projection onto the earth's surface. You'd determine it's location by what direction you turn to face it. The angle your head is pointing up (inclination) doesn't matter, just the angle your feet are pointing at.

If the moon has already set and is below the horizon, you would just find the shortest distance to that projection. If it has recently set, you would face where it was last seen. If it is close to rising, you would face where you predict it to rise.

On Mars you'd simply do the same, probably by using a phone/computer, this is what Muslims do nowadays. There are apps that show prayer times and the direction to face to see the Kaaba. An app like stellarium can show you the location of stellar objects.

If you somehow did not know where the Earth's general location was, you'd just pray in whichever direction you want. There's nothing wrong with that. Before the compass, Muslims used the sun to determine the angle to face towards. However, on an overcast day, a best guess would just be made.

On planes, Muslims just pray in whichever direction is convenient. If the plane is flying near Mecca, the direction would have changed by the time you finished praying either way.

The only requirement is that an effort be made, just the intention to face the Kaaba is enough.

The earth isn't flat, but Muslims "face" Mecca when praying from anywhere around the world. Same concept.

It's also worth noting that most ancient people knew the earth was round, including early Muslims.

Let's just hope religion is fully dead before we ever have to worry about this, shall we?