1) Static nat means that the public IP address correlates to a private IP address in the internal network. Dynamic usually means that either one public IP is overloaded (PAT) or multiple public IP’s are used in a pool and correlate to various private IPs.
ZS by default uses Dynamic (PAT).
2) You have to change the rules in the iptables, so that your external public IP address equals to the internal private IP of the vpn accelerator. All this would require a post boot script to change the configuration, as there is no such option in the web interface.