Nothing you said is correct or supported by the constitution. Just as an example the fed cannot enter a state for even benevolent reasons unless invited by the state first. The main problem after Katrina was the the Louisiana Governor waited way to long to invite any federal assistance in.
As I assume you are referring to White v. Texas (1869) in particular, or as one such case, that was just wrongly decided constitutionally by Amendment X.
The obvious reason was:
in the recent war between the seceded states and the present "reconstruction" thereof, is the highest court in the land now going to say the Union had no legal authority to resist secession, and thus operated under nothing but imperialism?
[Northern] Americans were too dead set that they preserved the Union, rather than they conquered a foreign sovereignty, as with Mexico 2 decades earlier.
In the end if a state wants to secede it will and nothing can be done about it. It has already been done by 11 states in the past and they later chose to rejoin the Union.