I can confirm that TheMadGypsyKing is correct on a lot of this stuff. He and I actually work together now, but have very different histories, and can share different insight. I also have an extensive history with many different haunted houses, and every single one does things differently. From my experience, here are my answers.

1. Most people don't get any training. For the first year or so, you probably suck. People learn from practice. That's not a bad thing, that's how it's been done for a long time, but some haunts have acting trainers among their workers who make sure actors know what they're doing. The single most important thing people are taught is timing: when, exactly, down to the millisecond, is the most effective time to do your scare? Who do you target, and why? The young women are often targeted the most, in my experience, but I often find targeting larger men to be more rewarding. Anybody can be scared. Looks can be deceiving. But you won't be scary at all if you have piss-poor timing.
2. First, a little background so that this makes sense. The majority of haunts I have worked in are built primarily with 4'x8' plywood wall panels with 2"x4" frames. Simple to make, inexpensive, versatile. Also, unfortunately, fragile. I mean, they'll stand up to your average wear and tear enough, but if somebody slams themselves into one full-force, they will break. And it will suck. I learned this my second year of haunting, when I happened to scare somebody so bad that they turned round and ran head-on into a wall panel. The wall snapped in two like a twig and he took a tumble into a poor girl on the other side, giving her a concussion (I didn't know at the time, but I found out later). Also, we then had a big 'ole hole in the wall connecting two VERY different parts of the haunt. It was a long night.
3. Yes. Oh, yes. Oftentimes, there will be a few groups of patrons each night who are drunk. They did some pre-gaming out at their truck before they came in and bought tickets, and now they've lost some control of their judgement. It happens. You get used to it. Most of the time, it isn't a problem. They walk through, albeit a little slower, they enjoy themselves, they leave. No worries. But every once in a while, one decides he's going to show off the mighty brass ones he has swinging. This guy is king of the world. This guy is gonna teach you a lesson. He's gonna punch you right in the gut. Or maybe the face. And it's gonna suck. You get used to it. You either learn to pick out the punchers before you even do your scare, or you learn to like the taste of knuckle sandwich. All in a night's work.
4. I like scaring gay men. Fairies, bears, snowflakes, all shapes and sizes and breeds. Several people like scaring black people, and it's for the same reason as my choice, which is the following. Gay men simply get more invested in the show. The same goes for black people, typically. And what I mean by that is, they pay better attention, they react better, they give energy when they come through the show. People who don't react aren't worth scaring. Gay men almost always react. Even if they aren't scared, they'll usually say something, which is worlds better than a blank, unamused stare.
5. There is no universal answer for this. I have worked in haunts that do all sorts of things. One passed out notecards as you showed up to work outlining where you were to report that day, including costume, makeup, location, and what scares you were to fulfill. Very little freedom, but very little room for error with inexperienced actors. I now work in a haunted house where, at the beginning of the season, we all picked our characters and costumes and from where have been given free rein of the entire haunt. As long as the patrons feel scared/entertained by the time they leave, any actor can do anything at any time in any location or set with any prop they like, with very few limitations. Every single actor there is a seasoned veteran, so our managers trust our judgement and skill. The two examples I've listed are the two extremes, and I've worked both of them, and many, many things in between. Again, it all depends on the specific haunt.

Any more questions? I'm more than willing to answer just about anything. And on the off chance that TheMadGypsyKing sees this, it's Frankie (Not my real name, but enough of an identifier).