I try one of the my interpretations. The method I am using is the following:
1) I assume that the author is inventing facts and episodes to contrast previous (Christian!) rumors and voices about Christ that were “Gnostic” and soundly anti-nomian in nature.
2) As it is often the case, the author has to invent facts and episodes to contrast also the rumors and voices that were anti-Christian in nature, and were anti-Christian in nature in virtue of the wrong recognition of ALL the Christians as an anti-nomian and “Gnostic” people.
3) As consequence of 1 and 2, the author has to find allies among the same not-Christians who were defaming the other Christians (not the Christians of the author) in virtue of the same reasons of the author.
The particular case in question is the following:
...is a Jewish attack against the Gnostic apostles (Ophites, Naassenes, Peratai) since their Jesus is the name of the Serpent, revealer and giver of the Gnosis (not a perverse being). Obviously, the Serpent is Satan for these Jews."He is possessed by Beelzebul," and, "He casts out the demons by the ruler of the demons."
The author shares with these Jews the accusation of “satanism” addressed against these Gnostic Christians adorers of the Serpent, but he would like to save in the same time the “good reputation” of the name “Jesus” as relating the true Jewish Messiah and only him.
As consequence the conclusion:
... means that the Jewish attacks against the Christians are forgiven, since they were simply identifying a false Jesus (the Serpent) as the true Jesus (the Messiah) and from this POV they were really innocent people.28 Truly I say to you, all sins shall be forgiven the sons of men, and whatever blasphemies they utter; 29 but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin"
But just the Gnostic “Christians” who adored Jesus as the Serpent, just them, are who “blasphemes against the Holy Spirit” and therefore there will be no forgiveness for them.
In the light of the premise and of the conclusion, how can we explain the middle part?
I think that here the author is taking the Gnostic words of the his enemies to use against them."How can Satan cast out Satan? 24 If a kingdom is divided against itself, that kingdom cannot stand. 25 If a house is divided against itself, that house will not be able to stand. 26 If Satan has risen up against himself and is divided, he cannot stand, but he is finished! 27 But no one can enter the strong man’s house and plunder his property unless he first binds the strong man, and then he will plunder his house.
THere is only a “strong man” in the Bible who is put to guard the property of a God:
Note the reversal of the values made by the evangelist “Mark”: who for the Gnostics had to conquer the false Eden of the Demiurge - i.e. the Serpent Jesus - after having defeated his cherubim of Gen 3:24, now he is degraded to Satan (the strong man) against whom now the Jesus Messiah has to combat.