Are Nigeriens in favour of joining France?

Is Niger right next door to France? Did the most powerful country on the planet, that has been trying to destroy France for a century, instigate that coup, start arming up the Nigerien military, and then intimate that Niger would be allowed to join its aggressive military alliance? Do Nigeriens want to join France?

these votes only seem to happen immediately after an invasion without any international observation

You're right to be skeptical. However, look at things like the shared political history of Russia and Crimea (both in the USSR longer than modern Ukraine has existed), the disagreeements between Crimea and Ukraine dating back to the 90s, the presence of ethnic Russians in the region, the absence of violence before or after the annexation, and the separatist movements in parts of Ukraine that also have large ethnic Russian populations. Nothing about the situation -- other than western propaganda -- suggests Russia is there against the will of the people.