Miriam -
Thanks for your helpful & quick response.
The 15+ business listings are individual dealership locations (ie Lexus of City Name & Toyota of City Name) are all currently grouped within one login so I (the dealership group's SEO guru) can manage all the information/implement best-practices from one single login.
We are debating setting up different @dealerdomain.com accounts if there is a SEO benefit. I am wondering how the Google+ & Google+ Local merging will happen regarding dashboard admin, so I'll wait to do anything until after that happens.
You confirmed what I was looking for, "There is no SEO benefit, at least not that I've ever heard of, of going with one account vs. multiple accounts in this scenario...."
And you are correct, separating out the Google+ Local from the multi-account setup is a big job if we decide to go the domain-based email address direction. We are currently weighing the pros & cons related to, "Google apparently trusts the domain-based email address to be more authoritative."
Any further insights appreciated.