One IP address to multiple hostnames or FQDNs

Yes this is possible.

Same goes for mutiple IP Addresses to one hostname

When you query a DNS server for rr.zahsystems.com you'll get back a list of IP addresses back. You can then choose to connect to one of them. Should the first attempt to connect get actively refused, just try the next.

Most browser will follow this flow, as long as the endpoints actively refuse TCP connectivity. Should an endpoint timeout, the resource will be treated as unreachable even though not all IP's have been tried

Since most applications (browsers included) are often only interested in 1 IP endpoint at a time and just choose the first available answer, you risk skewing the load between the target servers so that the first server gets all the traffic while the others might be idle.

To circumvent this, most DNS servers offer what is known as a Round Robin configuration, making the server alternate the order in which equally matching records are returned. Before load balancers were common place, this was an efficient way to load balance and somewhat implement fault-tolerance on network systems.