From reading this it sounds like you can get ethernet cabling into all the units ?
Best option is to use a single router and place a switch behind it and run a single connection to each unit. Wireless or other would be tenet responsibility. You could place a access point in each apartment on the end of this cable if you really wanted to provide wireless.
This should provide the best network between the ISP and the users. You still are limited by a single cable connection which many users can eat all by themselves. So it may still be slow, you have no way to control over usage by one or two tenants.
Most your other ideas require running wireless signal though the walls. This will be a support headache. It take a very good site survey in the first place to position the equipment but as thing change you will have to worry about complaint of "slow".
Be sure your ISP knows you are doing this it is generally against their policy to allow you to sell/giveaway their service. They of course have plans for a price that will allow this.
You may want to get a log server or something. If one of your tenants does something the IP address will trace back to your connection and it will become your problem to explain why it was not you that was **** bad things.
I have a 13 units apartment building with approximately 33 potential internet users, 3 floors. I use a cable modem (timewarners fastest cable service) I have type N routers wired from the modem.
I have problems with slow traffic and signal loss. I am not trained in computers or networking. I do have a grasp of the basic concepts.