Firewall failover cluster behind BGW320-500 with Static /29

I can't seem to find a way to pass our static IP block through the RG to our redundant firewalls which each need to have their own address and 2 that they share between them for failover. I am not able to get inbound traffic such as VPN to pass to the firewall.

Nope, you probably can't.  The Gateway has a one IP per MAC address rule, and your requirements don't look like they fit that.

I have tried passthrough with manual IP assignment, ant the best I could achieve was traffic originating from the LAN to internet works fine, but traffic originating from the internet is being dropped, even with the packet filter firewall disabled in the RG.

IP Passthrough has nothing to do with a Public Static block (other than changing how the SINGLE destination IP for the Public Static block router is configured).

It seems cascaded router option won't work because that binds to one device MAC address and may or may not still do NAT, which will again cause VPN to not work.

It will not do NAT, but it does bind one IP to one MAC address.

Why does at&t provide a residential gateway for business service anyway?

Your sophisticated firewall configuration is not supported by shared fiber Internet service like you have ordered.  It's a small business service, not an Enterprise Internet service.  You need to get dedicated fiber access, which will get you different equipment and better support... at an increased cost.