In other NT verses the forgiving of sins by Jesus is linked to Jesus healing people as a result. Verses 13 - 27 are about Jesus Jesus preaching and expelling demons, and about the apostles of Jesus obtaining the authority to expel demons.
Many religious people of the first century CE believed that having certain diseases and being demon possessed (being what people back then considered to be demon possessed) meant that the diseased (or evil spirit possessed) person must have committed a major sin. (For an example of this view, see John 9:2.) They believed that many diseases (and some cases even what were thought of as people becoming possessed by an evil or unclean spirit) were due to YHWH/God punishing people for their sins. They also believed that people committing certain kinds of sins would result in Satan possessing people (or in Satan sending a demon to possess people). As a result, people having such views believed that if God forgived (forgave) people of such underlying sins, then by means of God (and possibly through a human servant of god) the health problem and/or evil spirit possession would go away.
To me Mark
3:28, 29 is saying that Jewish worshipers of YHWH/God, and perhaps even all people, - except for those who blaspheme the holy spirit, can have their sins forgiven. Thus to me those verses are not saying that only the 144,000 (whether a literal or symbolic number of anointed ones of God) can have their sins forgiven. This idea (at least for those having faith in Jesus) is also expressed in John 3:16 - 17. However I think that according to Mark 3:13 - 30 Jesus and his fellow anointed ones (at least Jesus and his apostles) have authority to forgive sins and to heal people.
Verses 22 - 27 and 30 make the point that when one attributes an obvious work of the holy spirit (such as the expelling of demons) [according to the gospel of Mark], to Satan (Beelzebub) and says that the human being (whether Jesus or his apostles or his other disciples) used to expel the demons has an unclean spirit, then one is blaspheming the holy spirit.
In contrast no one of the governing body of the JWs, despite claiming to be of the anointed, claims to have the gift of the spirit (such as the power to perform exorcisms) or heal people's bodies by supernatural means). Furthermore there is no evidence at all of any of the WT's/JW's governing body members ever having gifts of the holy spirit. As a result, criticizing the governing body (even attributing evil to them), from a biblical standpoint does not amount to blaspheming the holy spirit.