Your best bet is to offload DHCP from the controller since it limits you to 512 addresses only.
On one of my customer networks, I have an external DHCP server and the controllers are in HA-FF mode.
The guest network is vlan 108 on both controllers with 2 different layer 3 interface addresses. We are using Clearpass for the guest captive portal.
For #1, yes you can create identical vlan interfaces but unsure of the dhcp pool. your guest VLAN will require an IP address for captive portal delivery.
For #2, maybe with database synchronization enabled but i cannot be sure, I know that client-states don't sync unless you are using 802.1x. When we performed a failover between controller 1 and 2, the guests, roles did not sync so they performed a mac auth since they already had a session. Again, our DHCP server was not on the controller for this case.
For #3, again you should probably look at moving your guest dhcp pool off the controller