yes the GALs have a limited lifetime. there are some hacks / brute force methods that you can use to read them, but of course not guaranteed. other than that would need to do reverse engineering with logic analyser and fingers crossed.
Maybe check what revision of motherboard seller was using ? wondering if its a rev 6 as well ? is your stock standard or existing memory expansions present ?
Any other A500 users nearby, I would definitely be tempted to try in a different machine with different CPU as well.
Silly question, your machine is confirmed working without the AT500 plugged in ?
With the UV light its all about light wavelengths. some objects reflect different things not normally present in visible light, plus camera lens is a little bit more sensitive to UV light and picks up things that the eye doesnt see. Ive got a 030 accelerator for my a500 that doesnt work, cant wait to do UV inspection
If you do end up returning it, why not just go for one of these modern boards ?
http://www.kipper2k.com/a500fastmem.html
