I never said we believe that she sinned. She was born fully human, like any one of us, with fallen human nature that desires (i.e. has propensity) to sin.
But we hold that she never did sin, not because she was somehow superhuman, but because she was bale to love God more completely than others. Which is why God, in his foreknowledge, chose her, as the suitable immaculate vessel, and a blameless Mother of God.
Because our nature is mortal, as a result of ancestral sin of Adam and Eve, Mary died like any other human being, even if she did not sin. Her nature was still mortal.
The assumption is Catholic teaching, therefore, it is consistent
Not really, not Roman Catholic. As far as we can go back into Orthodoxy, the East always mantained that she died and was assumed into heaven on the third day. The Catholic Church did not make it a dogma until 1950.
We also don't teach that Our Lady was superhuman. Immaculate Conception (celebrated today!) means that the same condition we gain at baptism -- freedom from original sin, -- she gained at her conception.