The firmware don't support cluster and the official website don't mention that support cluster mode anymore. So i would advise that you could use controller to configure them then turn off the...

Refer this FAQ http://www.tp-link.com/en/faq-492.html. I would advise that you could configure static ip for TL-WA901ND, and disable DHCP in AP. Then you try to access the web UI again.

You could use EAP110 lonely without using EAP controller. But some function you need to run controller.

EAP serial products don't have bridge mode. If you want, you could consider TL-WA701/801/901ND or AP300, AP500.

I think TP-LINK have mentioned that in website and You can only upgrade to the EU firmware after update. Why do you want to go back to original firmware? Maybe you need to contact TP-LINK support...

You could use Multi-SSID mode then could configure VLAN for each SSID. But it requires that your router support VLAN routering, the ip address is assigned by your router. Or you have L3 switch could...