The text you provide clearly does not support that interpretation.
No amendment shall be made to the Constitution which will authorize or give Congress the power to abolish or interfere within any state, with the domestic institutions thereof, including that of persons held to labor service by the laws of said State.
D-uh, the Corwin Amendment did not "legalize slavery everywhere in the Union." It allowed states to make their own laws regarding slavery, and forbade constitutional action to abolish slavery, i.e. it asserted "state's rights," what you guys want. It would not legalize slavery in Michigan or Vermont or give it additional protection in the free states.
Nor did it represent Congress's ideal intention with regard to slavery. It was a last ditch compromise effort to save the union. Moreover, the Corwin Amendment was an attempt to head off the Crittenden compromise which would have made slavery legal in the territories." Neither proposed amendment would have "legalized slavery throughout the country but at least the Corwin version would have left the non-slave territories free.
Whether the Corwin Amendment would have been ratified and what status an "unamendable amendment" would have are debatable questions with no easy answers. The supporters of the amendment widely regarded the amendment as simply reasserting the Constitution's provisions with regard to slavery, not as some radical change.
The Corwin Amendment certainly wasn't our nation's finest hour. It was an attempt to prevent the country from falling apart by trying to satisfy Southern demands for Constitutional protections for slavery.
If you are insinuating that Dred Scott was issued in 1861 then you are sadly mistaken. The decision was issued in 1856.
The Corwin Amendment to the United States Constitution, Number 13, Would Legalize Slavery throughout the Country.
Absolute nonsense, as you would know if you bothered reading the amendment to begin with. "No amendment shall be made to the Constitution which will authorize or give to Congress the power to abolish or interfere, within any State, with the domestic institutions thereof, including that of persons held to labor or service by the laws of said State." If slavery was legal in Alabama then Congress couldn't end it, though Alabama could have had they wanted to. But if slavery was illegal in Michigan then the Corwin Amendment did not mean that it was suddenly legal. Michigan could continue to ban it. The Corwin Amendment was, in its own way, no different from clauses in the confederate constituiton which established slavery in the confederacy and ensured the central government could not interfere with it.