Hello!
Thanks for your review =).
Regarding the admin-ajax.php that is a general wordpress page that is used by any plugin to do ajax requests (which are like requests to the website without the need of refreshing the page, in my case in the front end it is used to save the form information).
Although the name of the page has the word 'admin' it doesn't mean that it is doing back end admin/insecure requests, it is perfectly fine and secure (and actually is a best practice to use it) to do request to that page from the front end.
Here are a couple of links that could help in case you want to lea more about admin-ajax:
http://code.tutsplus.com/articles/more-tips-for-best-practices-in-wordpress-development--cms-21013
https://codex.wordpress.org/AJAX_in_Plugins
Hope this helps but if it doesn't please let me know =) (by creating a support ticket here please: smartforms.uservoice.com)
Regards.