Guys, especially teenage guys can get kind of excited about even small levels of physical contact from a girl. it is not an intentional or even a conscious response. I remember many embarrassing high school dances when I tried to scoot my hips away from a girl during a slow song so she wouldn't know I was getting an erection--tent dancing rather than full body slow dancing.
After a guy gets more used to the physical contact he becomes a bit more desensitized and won't automatically react that way as much unless the contact is prolonged or involves a little extra. Although getting an erection can still be somewhat unpredictable.
So sometimes a guy might get erect from a long close hug, but sometimes from nothing at all, especially in the teens and early 20s.
Not usually. It seems like a lot of girls think a guy gets a boner over everything little thing she does with a girl, even just looking at her sometimes. It's about asking, "Do girls get wet from hugs?" I suppose some do sometimes. lol! But I guess if my girl was wearing the sexiest perfume, dressed hot and we were pressed close while hugging... yeah maybe. :-P
It can happen. In fact, I recently had gotten a reaction just from imagining a good, long hug with a girl. It doesn't happen often (as I'm rarely hugged) but if I were hugged more often I figure it would happen more often. It can take a bit to get a reaction out of me but a hug can do it. A hug is an embrace, a way of just being close to a girl. That can evoke a lot of things from a guy, including what you have asked about.
I can get a boner from virtually anything involving a girl. Looking at her, her sitting on my lap (that is really embarrassing), dancing as well (embarrassing again), thinking of any girl can do it...so I'd say, yes! Hugs qualify.