Unless you have 12 unique IPs on the WAN link at the balance you will need to do port mapping as well as forwarding here, is that an option?
As in can you tell the iPad that device 1 is at a.b.c.d port 81 + 8081 for example where a.b.c.d is the public IP of your balance and let the balance map port 81 to port 80 on the inside. Device 2 would map 82 + 8082 and so on.
With regards to the port forwarding configuration at the balance is there NAT enabled on the PepVPN profile between the balance and the max? If you are can you re address stuff to get rid of it, as otherwise you would need to map the ports on the balance to the PepVPN peer IP of the Max and then port map/forward them again on the Max.
Assuming no NAT on the PepVPN and that port mapping is an option you just configure the port forwarding on the balance and tell it the “server” IP address is the device you want, repeat this for every device and port mapping combination, incrementing the external port each time.
Another approach to this which may be less work (and potentially more secure) would be to setup a remote access VPN on the balance and have the iPad connect to that, it could then directly access the devices connectd behind the PepVPN > Max (again assuing you are not doing NAT on the PepVPN profile).