I think the grace access can only be used if you are using the guests approvals which I believe it came out in version 2.7, otherwise that option would not be available. However, if you are not using any approval process then I think you can meet your requirement by redirecting the guests to the login page where they can see their credentials and use them to access the network. You can configure those settings in the "Registration Form Settings" inside the interested portal. Also, you might need to check that the username and the passwords boxes are ticked in the "Self-Registration Success Settings", otherwise they won't show up on the login instructions page.
Assuming the ISE nodes are configured in the same deployment, you can use both PSNs at any given time to serve the guest traffic. ISE PSNs work in active/active fashion. One key thing to keep in mind is that the whole guest session should be always served by the same PSN. For example, if the guest connects and gets redirected, the redirection should go the same PSN on which the session was initially created.
To ensure that, you can create two authorization profiles for redirection (which will then be associated to the redirection authorization rules), one for each ISE node by specifying the node IP or hostname. On both rules you can use the same portal, no need to create multiple portals. Another way to achieve this which I personally prefer is to create a single redirection authorization profile without specifying any ISE node IP or hostname, and then creating the IP aliases through CLI. The reason why I would prefer this method over creating two authorization profiles and two authorization rules is just because it is simpler and cleaner. In this case you just need one authorization profile and one redirect authorization rule.
Each ISE node should have its IP alias created, for example, on ISE 1 you create the alias "ip host < ISE node IP or the guest dedicated interface IP > < the guest portal FQDN >". The guest portal FQDN must be different between the ISE nodes, for example, you can use "guest1.company.com" on ISE 1 and "guest2.company.com" on ISE 2.
The DNS resolution should point to ISE nodes IP addresses, specifically the interfaces IP addresses that will be serving the guest portal. So on your public DNS management console, you just create the DNS entries and you associate them to the private IP addresses.
