I expect Finn had some basic concepts of the circle... but he SURELY should have known how very dangerous a Loup is. And if he had the circle concepts, he might want to make sure Kim knows this thing he becomes is a major ass-kicker.

My point, how is it he didn't see through Kim? 

Don't have much to add to most of your arguments, except about the chicken dinner reply.  While
yeah, most young people are arrogant and a bit know it all, Kim has a lot in common with Molly here.  It could also be why unless kids with talent are taken under the wing of a wizard they fall easy prey to the temptation of using their talent to do stuff that ought not be attempted except by professionals, or if the professionals don't do them, there is a reason why.  If they pull it off it leads them to think they can use it to more stuff, next thing you know you have a warlock on your hands.

why didn't Finn simply ask Kim if she knew of any wizards in the area that might help him?

She compares herself to Harry because he isn't that much older than her and doesn't take into account, experience, training, and talent level.  At that point confidentiality should have gone out the window, but Kim let her ego and perhaps her own ignorance get in the way.

Yeah, but "considered armed and dangerous" is often enough to get the suspect killed.

Harry expected that the kind of summoning-circle she was proposing would be taken as proof of guilt, and she'd be executed for it.

kind of circle was needed to contain him, no, I think he knew a lot more than
No, then who built it there in the first place? Why? How did Finn know about it?That's a possible way, if that is what he did, he must have hauled it all over the world with him.

Harry was going to try.. Materials are important,and maybe just just useless bling in this case.  Remember I think it was after Harry was made a Warden and had a regular paycheck, one of the first things he did was upgrade the circle in his basement to silver, I cannot remember what it was made of before, but I remember him saying that the silver made a stronger circle.

Here's one.... they say archangels - could this circle, perfectly drawn, hold Uriel? a Mother? I tend to think humans can't even channel that sort of mana. Maybe Jim should have left out arch-

  It is the second book in the series, Jim could have changed his mind, but then again the circle has never been tested on page against the like.  Uriel did shrug his shoulders at the pitiful circle a wounded Harry made to try to contain him in Changes, but that wasn't the same kind of circle.

It really wasn't a trial, it was more of a kangaroo court, that is what Harry objected to. That is what he insisted on for Molly, a real trial at the end of the book. 

  Harry hints that knowing how to build such a circle flirts with the dark arts.  It isn't just about having one of these circles in your back yard, but what you could call up with it. She wasn't a minor wizard, she wasn't even an apprentice, she had a little talent.  The speech Harry gave her about the circle, what it could contain, controlling such a thing sounded a lot like the one the Merlin gave him about young would be warlocks, the immense amount of danger, deaths, if they were allowed to live and something went wrong.. Which it did, and Kim and a lot of innocents died.

   Why didn't she explain about Finn?  I think Harry would have gone with her and tried to help Finn.  Kim wanted Harry's knowledge, but she wanted to handle it herself, she wanted the credit.. In other words she was beginning down the road to the dark side, I believe the White Council would have seen it the same way.

I have to go back and reread, but the circle she wanted to build, and the type of monsters and demons it was designed to contain, it was illegal or drew some interesting assumptions from the Wardens.  In other words if you needed to build that kind of circle to call up whatever or contain whatever, you were up to no good.. Remember his buddy Morgan?  At that point in time Harry didn't know he was really looking out for his welfare.

I really wouldn’t be surprised to find Chauncy working with Nameless/Cowl. How else did the summoning rite for Chauncy get out into the mortal world?

St. Patrick (almost certainly) did NOT enact the MacFinn curse.