I would say question is somehow wrong, as thing work a bit differently:
1. You usually don't want to use DHCP service on ZD, it's there just for small and simple implementations.ZD can provide DHCP service only for subnet it is connected to.
Important to know that ZD doesn't need to have IP in any of client subnets, ZD needs only to
communicate with APs management interfaces in management VLAN (except if you use authentication portal, than it must be reachable for clients). Anyway, keep it in management VLAN, separate from client VLANs and client's traffic, and don't mess with dual-homed configurations, it's mainly not needed.
2. Client always can use gateway only from VLAN to which it belongs. To separate clients from management VLAN/Subnet you can create as many client VLANs as needed, each with own IP subnet, IP pool on Windows server, own gateway, and assign this VLANs to SSIDs. Clients on different VLANs use different gateways to access Internet, typically routing is provided by L3 switch, router or firewall.
3. Important -- if DHCP server isn't on the same subnet as client, you need to enable and configure DHCP helper on switch. DHCP helper forwards DHCP requests from client to DHCP server on different VLAN, and forwards answer to client. Without helper DHCP will not work. outside of same VLAN.
4. This is not VLAN pooling. VLAN pooling is a feature used in high density networks, when SSID has 2 or more VLAN assigned to it (each with own IP addressing, gateway and DHCP service), and wireless clients are distributed between this multiple VLANs (in accordance with they MAC hash). It is mainly used for public network on huge venues.
Hope it helps,