@Tony H.
You have a lot of info to cover here so I apologize in advance if I miss some stuff or go off in an unrelated tangent... lol
1) You need to check with the city zoning regarding "Non-related individuals that can live under the same roof". I've only dealt with this in R1 which is NOT for duplexes. Most cities will allow 4 - 5 unrelated adults to occupy the same residence, anything more is not allowed. That being said, most places really don't care if you have more people in. Or if they do, it's really not easy for them to find out. So, if you play your cards right, make sure you know the neighbors and treat them well, you'll probably be fine renting out more than that.
2) I really hate using numbered rooms or letters. It's really cheesy and unappealing, not to mention it's difficult to remember which is which. I like to label my rooms by a defining characteristic: Master, Large room, Side room, west wing, etc. Then when you are showing the room, you are showing the "West Wing Room" instead of the "Room E". A little psychology goes a long way. As for leases, it really depends on how you structure the agreement. If you are renting to 3 people that are moving in together, they all go on the lease together and are responsible for the lease TOGETHER, so if 1 or 2 drop the ball, they are all legally responsible. Personally, I don't like this approach for the room rental business model. I go with individual leases that ONLY pertains to their room. They legally have access to the common areas and are responsible to keep stuff clean and orderly, but they only have legal lease on their room. I do this because often times people move out and you would have to find a random person to fill that may or may not get along with the others, etc. and it's hard to get them to share a lease with someone they dislike. I personally only do month to month because if I don't like someone there, I don't renew their lease. No need to evict, etc. just don't renew the following month and give them 30 day notice to leave.
3) Fortunately I have not had the need to evict anyone (I manage almost 30 rooms for the past 5 years). I've been able to just get people out since it's month to month. I tend to deal with college kids, so it's very easy to get people to leave. If they think nobody in the house likes them, they feel the social pressure and just move out on their own lol it sure beats going through legal.
4) I think you are barking up a tree that you probably shouldn't. My average rent per room is about $750/month (I include utilities, furniture, house supplies, etc.) and I only collect a $300 deposit. I've had to keep a deposit twice, once because they skipped out on rent and moved out middle of the night and the other was because the angry Ex-BF bashed in the door lol the repair wasn't even $300 but she felt so bad she told me to keep the deposit as she left. You can use the deposit for anything broken IN THEIR ROOM. Unless they are all on a master lease, you cannot charge everyone for damages done by one or two people. In my experience, when you do room rentals, you make so much more money than a regular rental, you should NOT BE CHEAP. Tenants will break things from time to time, they will clog common area toilets, they will break the washing machine, etc. etc. If you start to nit-pick and push costs on everyone, when in reality it's only a single person that ever does all the damage, then you will lose good tenants. There is a saying in business, "Don't trip over dollar bills trying to pick up pennies". Don't do this lol. Example: I clear about $2k in one of my homes each month after all normal bills are paid. I probably get 2 - 3 plumbing calls per year ($60 per trip), 1 - 2 appliance repair calls (about $100 per call) and a few minor things here and there. If I rented the house out normally, I would be negative cash flow, but I could charge the tenants these fees. Instead, I suck up the $400 extra in expenses per year, put a smile on my face when I head to the bank and learn to have thick skin :)
I hope this helps, let me know if you have further questions. I would be more than happy to share my knowledge in this area.