Is there any reason you are installing 2.121 instead of 2.212? Are there any debug files (check in your cache folder for Zen Cart 1.5.0 / log folder for Zen Cart 1.5.1)?

1) You say you are "building" the site in a subdirectory? The root of your website looks like a Zen Cart installation... And there is an extra "/"'s in the Zen Cart URLs... You will want to check your configure.php files. This typically indicates an extra "/" somewhere in that file.

2) This installation on the root of your site also appears to not be configured to issue a 404 for invalid pages and again shows a 500 error... which may indicate an issue with your webserver environment or your .htaccess file. You will need to look in your server logs to see what is causing this error.

3) The installation for "Ultimate SEO URLs" does not do a full check to see if files are correctly uploaded, the contents of the files are valid, or any necessary merges have been done. From the behavior you are seeing I venture something may have been missed when installing "Ultimate SEO URLs" - probably one of the html_output file changes. Once you have addressed 1 & 2 above, download version 2.212 and redo the installation completely (one step at a time).