> let's imagine I create a company in Western Sahara and launch a satellite from there. What would happen?

If you're launching out of Western Sahara, it strongly implies (a) you're doing something naughty and (b) procured your equipment through illicit channels. The reasoning behind (a) is you'd spend vastly more on (b) trying to do this out of Western Sahara than you ever would on compliance in an ITU-member nation. The reasoning behind (b) is export controls.

I thus presume the response would be military (i.e. detainment, not blowing things up). This would probably be the responsibility of whomever you're a citizen of, or whomever gets angry enough about you. (In Western Sahara, probably NATO, Algeria or Morocco.) The more realistic concern is someone bribing officials in an ITU member state.

If you actually launch without, somehow, anyone knowing about you and having "conversations" with you beforehand, then it depends on the direction in which you launch. If you launch prograde, it'll most likely be shot down by Russian interceptors. If you launch retrograde, it'll most likely be shot down by American interceptors.

A polite conversation with host country leadership?

I'm fair certain the "authorities" in that area of the world would not be "polite" under the circumstances that hadrien01 postulates.