Я лечу в Россию часто
Question: Wouldn't this sentence theoretically be possible IF Antonio often often flies to Russia (from Ukraine, for example), but he always takes the train (or an automobile, or some means other than flying) when coming back from Russia to Ukraine?

In other words, you'd use the unidirectional form я лечу because each flight is a one-way trip, even though you make this one-way trip habitually. (And each train trip from Russia to Ukraine is, likewise, one-way.)

But "Я летаю в Россию часто" (multidirectional) means "I often fly to Russia and back".