Hello Galanopd ,

Dolphin to UNA migration script can work with Dolphin 7.2.x and upper, you don't need to uprate 7.3.4 up to 7.3.5 for migration.

Looks like you just set wrong path to Dolphin inc/header.inc.php file and it was not found by migration script. The simplest way to find Dolphin path is to open inc/header.inc.php of your Dolphin site and copy value of $dir['root'] variable and past it to Migration Tool -> Path to Dolphin Script. 

Una Migration tool uses only database and media files locations, thus even if you Dolphin has broken files, but header.inc.php file contains correct database and files info, media files/folders are not messed up, database is not corrupted, then Migration tool should work fine.