I saw that there is a script to switch from apache to Ngnix, my question is is there one to leave apache again?
The other question is, in a clean installation, is it possible to leave ngnix as a web server? In this case, do you remove Apache from the installation and keep php-fpm or do you need to do something else?
Apache is never alone, if you install Apache, Nginx will be installed as Proxy. So, the answer to your question is, no, there is no way to remove nginx or comeback to nginx+apache (at least not a way supported by Hestia).
Just remove Apache from install options (--apache no) and keep php-fpm and thats all, you will have nginx+php-fpm.