If your DHCP server is handing out leased addresses to other clients you might have bigger problems to worry about. For example if you expect a lot of churn on the network you should be using much shorter leases times.
How can a lease be "unused" if it hasn't expired and the client that requested it never released it? Are you saying that the DHCP server pings (ICMP or ARP) each address and re-leases the ones that don't get a response?
Well actually the result of "running out" of addresses is undefined and up to the individual hardware manufacturer, so theoretically they could do whatever they want. A high-traffic WAP might want to simply boot the oldest lease if they can't assign more than a certain number of addresses.
I don't think this is true at our office (running a Cisco DHCP server). We get conflicts when putting a device with a static IP in the dhcp range, and I'm almost sure that they respond to ping.
