1) power store does not use transactions, but data integrity settings can be enabled on the MySQL server by setting the SQL_MODE to STRICT_ALL_TABLES in the server settings files
these links talk about data integrity settings in MySQL:
You may also want to change the storage engines used for the tables to InnoDB
2) Error handling is inherited from your php settings and not controlled by our code... it is fully customizable:
3 & 4 & 5) SSL can be installed any time, before or after the installation, but it will need to be installed to the same domain for power Store to recognize it and redirect the user the HTTPS:// version of the checkout pages.
6) if you access the index.php file at the root of the site prior to installing, it will direct to the install/index.php file.