Nat – Block of 64 public IP’s, how many usable

1. If I understand your question correctly, the NAT router translates addresses as data moves from the "inside" to the "outside." If traffic doesn't pass through the router, there is no translation.

2. "Control plane" and "data plane" are conceptual terms. There isn't always a correspondence to hardware or software. That said, forwarding traffic is the function of the data plane.

3. The terms ALG, firewall, layer3 switch, etc have no fixed definition. They get defined by the manufacturers and can mean whatever they want it to mean. But generally speaking, an ALG often performs the same functions as a firewall.

4. TCP/IP can't read IANA specifications, so a port is a port, no matter what number is used. They are all handled (and translated) the same way.

The two kinds of NAT your question is referring to can be categorized as destination NAT, and source NAT.

Destination NAT will typically change a connection to your router from the ISP direction to a destination target that's inside your network. It's also commonly referred to as port forwarding. This lets you expose a service that would normally be inaccessible inside your private network.

Source NAT will change a connection through your router from your local network to a source target that is the router itself, allowing multiple hosts to use a single public address.

NAT can perform other translations, but few others are useful except in niche cases. Despite the phrasing in your question, a source NAT that you describe is not helpful. Using a global address that is not functional on the router will have no good effect. No other device or protocol would keep track of that, so all replies will be forwarded to the device that should have that IP, which would know nothing about your setup and promptly drop them. If you want to use a different IP, get it set up and functional on the router first.