Domain Access vs. Multi-Site vs. Sections -- Secondary Sites using Directories rather than Subdomains

I've got what seems to be a unique situation (based on searching the site for the past day or so) and am hoping someone can point me in the right direction.

I'm setting up websites for an organization that will have an umbrella site at site.org, and needs two additional sites at site.org/site2 and site.org/site3. We don't have the option of creating subdomains. These separate sites need unique themes and content, but would ideally share content types and users.

I was planning to use a multi-site install with prefix-based user table sharing, but found several posts that seemed to indicate this was not advisable behavior (the phrase "you're on your own" was mentioned I think). Since all sites would be on the same domain, this would effectively create SingleSignOn behavior.

Now that I've read more about Domain Access, I like the idea of being able to share specific content across all three sites while still keeping certain pieces unique, but it doesn't look like DA allows the use of subdirectories rather than subdomains to differentiate between sites.

Can anyone suggest the cleanest method to minimize redundancy across the three sites while maintaining the system's integrity? I'll be leaving this project in the hands of a PHP novice and don't want things to break in the future because I used incorrect methods.