Connor was also being protected by his mother. As nasty as it is, if he hadn't been in a castle, they could have just stabbed him and ended the problem then and there.
As for being possessed unwillingly, the only time we saw that was in Wilhelm's village and that took a lot of effort on the part of the demon.
The fact Connor had access to blood magic is another thing we can blame on Jowan.
Nice job, genius.
Stabbing abominations is more difficult than you think. It's what you need to do, yes, but it works only if you can reach them. And they don't really need a castle to make that difficult. They can make things explode.
It takes some effort for a demon to possess unwilling people, but it can still happen. Especially if they do stupid things. And we have from a Codex written by Orsino, who might be expected not to come up with pro-Templar propaganda, that even the First Enchanters are not immune.
As for how Connor got possessed, my point isn't how he get possessed, it's that he did.