Google+ now allows you to set-up multiple admins, so that might help your main issue of who should set up the account.
Saying that though, since you're dealing with local businesses, I would suggest that you use the login info/password that they used when they created their Google places page(now google local) if that's applicable. That will allow you to go continue without going through the headache of having to contact Google and claim the place. If you do use this login/password, you will be able to create a personal account for the client which they then can use to share the businesses' page updates and links.
Basically, you can now use this personal page to help promote the business. For instance John is the owner of pizza King, he can not only share Pizza King's page info but +1 it on occasion and also add value to the conversation going on, within Pizza King's page.
Using John's page will also allow you to +1 and share the information in case you're worried about it being associated with you(the marketer).
For easy access, you can also set up your Google+ page as an admin, to the business page for quick updating and messages.
I hope that helps.