@Beth131 - you're jumping the gun a little.
1. You are not responsible for the actions of your guest. You are indemnified from their illegal activity unless you go out and buy the alcohol - which you didn't.
2. You may have to tell them again about the dog. It's awful to have to monitor this behavior, but dogs are compelling to some folks and think it's no big deal. I understand that it's a big deal to you and your family because you will be left with the aftermath of any "treats" the dog gets. Once you tell them, "The dog has a really sensitive stomach and will poop in the house. It really sucks cleaning that up and if he does it downstairs, well, that will suck for you guys as well. Please don't feed him or let him down there just in case." Tell it like it is - they don't want to clean up or smell that kind of mistake!
3. Assuming the friend needs to come in the front door, go down and ask to speak to the booking host in private. Remind him that you said only 1 more person could come by and that you are not covered by liability insurance for any one else to be here. It is for their protection as well as yours and they'll need to head out to hang elsewhere (this is not a lie, you are not covered even for that extra person by Airbnb but you might be under your own homeowners). If they give you grief, tell them that you have to be firm here and that you are sorry but they can't hang out with non-Airbnb guests at your house for liability reasons. Recommend some joints where they can hang out together with their underage friend (Starbucks maybe? Or a local watering hole that doesn't check IDs?)
4. Don't speculate what the air freshener is for.... some people are really weird about bathroom smells they leave behind. I once had a guest who brought her own air freshener and sprayed it everywhere - it was weird - but, well, we deal with other people's weirdnesses all the time. It's your house, if you have a no smoking policy, you have a no smoking policy and feel free to monitor if you think they are smoking inside. However, after reading your listing, you have "smoking permitted" as an amenity - you need to remove this because that is directly related to smoking inside - not on the property. And If you don't want vaping, pipes, or cigars outside, you need to say so explicitly because marijuana is legal in your state. Or just say no pot on the premises.
The key here is to communicate, communicate, communicate in a professional way. If they do start violating house rules or policy, call Airbnb. They might really be okay young people and are new to traveling too. Try not to "parent" them - something I'm ashamed I do from time to time.
Good luck.