So, if I understand your logic, the South was not wrong to secede, but only in the manner in which it did so.
If that is correct, then you do not have a problem with secession in and of itself.
I do not have a problem with secession itself only with the reasons for the south’s secession as well as their method.
Secession by force of arms was a losing proposition from the get go, the south didn’t have the money or arms to make it work to begin with, and they could have successfully seceded had it not been for the question of slavery. It was an evil that they could not justify to sanctify their cause.
It is my opinion that the south without slavery would have been able to win it’s independence, or would never have needed to secede in the first place.
If you are going to secede you must do so for reasons that are above reproach especially in the eyes of those other countries you may appeal to for assistance.