Is an act that is "not permissible" considered a sin, and is the ruling of "not permissible" less severe than the ruling of "forbidden"? And what is the ruling on licking the anus from the outside, and inserting a finger or other objects into the anus for foreplay?

There is often no difference between "not permissible" (لا يجوز) and "forbidden" (حرام). The ruling on inserting a finger into the wife's anus during intercourse, and the prohibition of a man inserting his finger or his wife's finger or other tools into his own anus, all have previous fatwas. As for licking the anus from the outside (the buttocks), it contradicts proper manners and sound human nature, but the text explicitly prohibited anal intercourse with a woman.