1 - There's no one command way to do it.
# Install Webinoly with no stack (0 option)
wget -qO weby qrok.es/wy && sudo bash weby 0
# Change PHP version to 7.2
sudo stack -php-ver=7.2
# Install LEMP Stack
sudo stack -lemp
# Create your WP site
sudo site domain.com -wp=default -cache
Redis and HTTP2 are already enabled, just install and activate the Redis Plugin.
2 - The way described before is the best way and maybe the fastest.
3 - Nginx Mainline is on the roadmap and will be available as an option in the next major release (1.4.0).
4 - To uninstall PMA, sudo stack -pma -purge
5 - How frequent you do your backups is up to your needs. Normally I do it everyday in the night using duply and duplicity, both tools are already installed with Webinoly.