Let's cut these guys some slack.
There's no indication that the Pharisees in this particular situation are the prideful types that Jesus excoriates in Matt. 23. They're listening to His teaching.
When Jesus forgives the paralytic's sins, their reaction is perfectly in line with the teaching of the Tanakh: only God can forgive sins, and anyone who claims he can do it is calling himself God, which is blasphemy.
The cognitive shift that must take place in the Pharasaic mind is not that someone other than God can forgive sins, but that since only God can forgive sins, Jesus must be...the I AM in the flesh. Those Pharisees who can grasp this (e.g., Nicodemus) become children of the Kingdom.
I would be the first to defend the Pharisees as whole. I think Christians in general have come to believe ALL of them were out-and-out villains and are too quick to assume they were all like the ones we see fiercely opposing Jesus elsewhere in the Gospels and, later, the apostles in the Book of Acts. The majority of them, however, were probably sincere in their religion and, as you noted, some like Nicodemus even later became Christians.
As for the Pharisees in this particular episode, we aren't directly told why they are present to listen to Jesus. Maybe they were there as sincere enquirers, but we know from other passages they would also often come to find some incriminating word from him upon which they could accuse them.
I don't think that, based on the known motives of some of the Pharisees in the Gospels and Jesus' response to these particular Pharisees perception that he was speaking blasphemy, it is unreasonable in this case to conclude that he is addressing their rush to judgment of his actions here. Then again, I don't see your conclusions as unreasonable either.
Who but God alone can forgive sins?
That's a legitimate question which they had not only the right but the duty to ask. We also have the right and duty to ask that kind of question when modern-day people claim to speak for (or even be) God. Someone might come along claiming to have had a new revelation*; we are in the right to question his legitimacy.
Jesus backed up His claims. His physical healing of the paralytic was the answer: "You're right! Only God can forgive sins. But guess what, guys ..."
*Catholic Caucus, so I'll leave it at that.