Malin, first, this is NOT a direct support board and it shouldn't be thought of as anything more than a volunteer-based community support community. People like myself (Arubans) do not spend our entire day waiting for or looking for responses to follow up on. Well, maybe Colin does. So no one is ignoring you, sometimes we are working in the background on your behalf while doing our regular job (none of us get paid to haunt these boards). Hope that makes sense.
That said, I spent some time yesterday discussing with engineering on how to make this work with IAP. For controller-based solutions, it works fine because we can configure the number of IPs allowed per mac address on the WLAN. Unfortunately IAP does NOT allow this modification to the WLAN stack, so the only application that will work in your case is to use 501 as a wired client bridge for a single device with mac cloning.
If you want to get another IAP-205, that will absolutely support mesh bridging to bridge the network for however many devices you have on the remote IAP. There are numerous pages on setting that up. My apologies for that not working.