An update on this: I found a solution, but had to use the unleashed firmware.
So I pretty much gave up on the standalone firmware as it seemed incredibly buggy. Like I mentioned before, with the reverting settings and seemingly no clear documentation about what each setting really did, it seemed like this was a no-go. Even with Robert's helpful reply I guess I didn't make it very clear exactly what I was trying to do, but the suggestion of having all VLANs go through the back haul port was not what I was after.
I was able to get this working by waiting for the 2007 version of the unleashed firmware, which as promised, delivered VLAN port management for the H series APs. The documentation for this by the way was phenomenal. Read over it really quick and instantly knew what settings needed to be changed. Kudos to whoever or whichever team wrote it.
I simply had edit the group for the "Access Points" section, go to the "other" tab, select the drop down next to "Model Specific Control" and choose the H510. A "Edit Port Setting" button then magically appeared which allowed me to modify the ports, associated vlans and all that good stuff. I changed each port to "Access Port" then gave each port a unique Untag VLAN ID which doesn't live outside the H510.
Once set, I went to the wireless SSID's I created and edited each one to be part of the associated VLAN I made on the switch earlier (advanced settings, WLAN Priority). So WLAN 1 is tied to the "back haul" port since I'm powering it over POE which is plugged into an upstream switch on the 192.168.0.x subnet. WLAN 2 is tied to LAN 1 (which is VLAN10 on the H510) and the 192.168.1.x subnet (another switch) and WLAN 3 is tied to LAN 2/VLAN 20 and the 192.168.2.x subnet. The upstream switches and devices are not configured for VLAN ID 10 or 20, these only live on the H510 and only act as a means of isolating SSIDs to the respective ports.
When you connect a device to WLAN 1 SSID, it gets a DHCP address from the DHCP server sitting on the 192.168.0.x subnet. When you connect to WLAN 2 you get a 192.168.1.x. So on and so forth. No clients on any of the WLANs can communicate to the other subnets, unless of course I build routes further out on the network, but the point is the H510 doesn't bleed anything.
Hopefully this helps someone else. I know it's a unique setup, as people seem to only use the ports for a wired client device, but here is how you can set it up to act as a wireless bridge to separate and distinct wired networks.