For one thing, though Thomas and Murphy visits Harry in the island, it is clear they don't do it often. My impression both Thomas and Murphy only visits one time each. Though my impression can be wrong, it would not be a mistake to say that neither Murphy or Thomas know much about Harry during that year especially after Mab sabotage Harry's communication. Like what Mab say at the start of book 15, a bit of a push here and there and suspicion could easily breads. In other words, even if Butters did ask Murphy or Thomas about Harry, I doubt any of them could say or do anything to convince him. In Butters place, I wouldn't be convince either.
So your claim here is that Thomas or Murphy
did tell Butters about the parasite, and he just didn't care about the consequences to Harry if he left the island? That may make Butters appear less stupid, but it doesn't make him more sympathetic.
And the number of times they visited is irrelevant--for Thomas especially. If Butters wants to get a message to Harry, he needs someone who knows where Harry is and can get to him, and Thomas is, so far as I am aware, the only person Butters knows with a boat. And both Thomas and Murphy know about the parasite because Harry told them at the end of Cold Days.
As for Bob knowing about Harry returning with his entire soul in book 13, well, Harry can be ressurected if his body is found. But the fact of the matter is his body was never found. They dont even know if Harry's body is still intact. To make matters worse, Harry is missing after the battle with corpsetaker. The logical conclusion is Harry's ghost is destroyed during battle. Since Harry come back with his entire soul, it is only reasonable to conclude that Harry is obliderated , spirit and soul. He is deader than dead. He won't have an afterlife let alone come back from the dead.
If this were happening during Cold Days, I'd agree with you, but the fact is, the timing doesn't work out. It goes:
Ghost Story--Bob realizes that Harry's body is probably still alive, and is confident when telling Harry that Harry can get his body back.
Cold Days--Bob meets Harry and confirms that it's definitely him, even if he's still the Winter Knight.
Skin Game--Butters has apparently decided that Harry is not Harry.
You see how that's a problem?
It is not that Butters wanted Harry know all of their problems even when Harry is at Demonreach, but in Butters point of view, Harry should have innitiated contact himself. Harry is the one who should reach out. As readers, we know Harry can't do that because the parasite in his head, but Butters don't know and suspicion has more than a year to breeds inside Butters's mind fueled by Bob. When book 15 comes along, any explanation is already useless because suspicion already sets too deep. Any explanation sounds like an excuse at that point.
Then Butters is an idiot who doesn't bother trying to acquire information. Butters ought to know about the parasite if he were using his head, and he ought to realize that, given Harry is the Winter Knight, he might not be able to reach out.
Now, Harry being the Winter Knight
is a good reason for Butters to be suspicious, especially if Butters thinks that Harry is either being deliberately isolated to manipulate him or that he is so much under Mab's control that she can convince him not to reach out to his friends, but I've never claimed that Butters' suspicion was the problem, only his reasons for it.
And don't talk about soulgaze. Harry has mentioned again and again that soulgazes are dangerous. The last time Butters soulgaze a wizard, it is freaking Grevane. We can be sure it is far from pleasant. If Harry is truely corrupt, soulgazing Harry may turn out as bad as soulgazing Grevane. Not even Harry dares to soulgaze just anyone. When Nicodemous offers a soulgaze to confirm his sincerity in book 10, Harry flinch away ASAP. In other words, it is catch 22. Without confirming that Harry is still the old Harry, soulgazing Harry is dangerous. But if Harry is already confirm to be good, there is no need to soulgaze anymore.
Yes, soulgazes are dangerous. But Butters acknowledged himself that telling Harry his suspicions was dangerous, and he did it anyway. And I'm not saying that this is the solution for everyone--but given that Butters is supposed to be the True Wielder of Fiddelachius(TM) and that he's had it explicitly pointed out to him that his suspicion has a good chance of contributing to someone losing their soul, I'm going to say that
he particularly doesn't get to use "it's dangerous" as an excuse not to do it.
(As an aside, I always thought that the reason Harry flinched away from soulgazing Nicodemus had more to do with not wanting to deal with the possibility that Nic was telling the truth than it did with soulgazes being dangerous.)
Murphy swingging from trusting Harry to not trusting Harry to back to trusting Harry again however, that is explanable in my opinion. She herself is probably torn. Like I said, after Harry's assume death at the end of book 12, what Murphy suffers is regret and uncertainty. So her state of mind in book 14 most likely is not stable. Which explain why her actions is rather contrary during book 14. If she trust Harry without confirmation, it will be reckless and irresponsible. It could endanger everyone and probably cause a disaster. If she choose not t trust, what if Harry is really Harry? How would Harry feels? She don't want to hurt him. Without having a confirmation, it is really wrong whichever she choose. Now you see why Murphy waver back and forth like that in book 14.
I can understand why she might be wavering. I have a much harder time accepting that she was only wavering when it wouldn't cause actual problems with accomplishing stuff, especially in conjunction with other stuff that I see as manipulative.