Custom Firmware 1.9.2.7-6b is latest ?

[Antiloop]

This would mean Oleg's Firmware are 500g based and would work also on the WL-HDD but WL-HDD features like automatic-USB-Copy-to-HDD wouldn't work, correct ?

If I understand the serveral Informations correct:
1. I could flash the OLEG Firmware under the Web-Gui of the Original Firmware
2. Test if I like or not
3. If something is missing I could flash the Original Asus 1.2.3.4 Firmware under the Web-Gui of the custom OLEG Firmware

Is this right?

first question -> yes as they are not part of the wl500g firmware (500g is nothing)

q1 > No
q2 > yes
q3 > No

you have to use firmware restoration util (supplied on cd or asus download site) see manual or search on how to use that