First of all, you are absolutely right to go to Mainz instead of Frankfurt!! ^

Rather than buying single tickets from FRA to Mainz and reture (€4.10 each), buy a "Tagesticket" (all-day ticket) for that route. The 4-Zone Tagesticket will only cost you €8.00, and it is valid on all S-Bahn and RE trains, as well as on all busses and streetcars in Mainz. So that's the better deal.

Both the Main Station and Römisches Theater Station are in the middle of town. There are sidewalks on all streets - and lots of traffic lights to help you cross. No worries here. Google always puts that warning on a pedestrian itinerary. Try Times Square to Grand Central Station in New York...