Your point 3 is complete bullshit. Crimea was part of Russian empire since 1783. During that time Ukraine was also a part of Russian empire - mind you, not autonomic one. After empire collapse in USSR Crimea administratively belonged to Russian SFSR up to 1954, since when it was a part of Ukrainian SSR. So it’s a part of Ukraine for the last 60 years.
It was part of Ukraine, and republics of USSR formally could leave it (not that anyone would allow that though, due to the legal trickery). So that point 3 is not true.