I would configure the guest-SSID to be assigned to your guest VLAN (99) and don't do any routing on your switch. Leave the routing to the Bbox and just put your clients at layer 2 in that VLAN.
From your switch config, it looks like you have many vlans tagged on port 47. I would remove all tagged VLANs from that port 47 and just have vlan 99 untagged, assuming this Bbox is a (similar to) consumer router that offers DHCP.
As soon as you start routing the guest traffic on your switch, it will just follow the default route and get to your corporate network (and probably have access to corporate resources).