Mariner and Mercury are exactly the same company and same engines with different labels. http://www.barrus.co.uk/marine_division.php
It is common for outboard engines to be rebadged versions of each other. In some cases it literally is just the stickers and paint that changes, occasionally there might be subtle changes and sometimes its more significant.
I think you are right the Tohatsu is also the same engine as the Mercury/Mariner at this size. All though you don't know the name Tohatsu they are part of the Nissan group, and indeed sell their outboards in other parts of the world under the Nissan brand. For some reason they seem to be slightly cheaper that Mercury/Mariner / Yamaha / Suzuki / Honda / Johnson/Evinrude, but they are generally reasonably well respected.