In re; Removal of Snape other than by death, I feel sure you would find this character would just have to be dealt with yet again, after more evil work, if on his own this time.
The parallel would be Treebeard letting Saruman go, only to have "Sharky" show up having despoiled the Shire, and the someone with more justice than mercy, unlike Frodo, putting an arrow through his black, prune shaped heart!