I configured SPNEGO in EAP6. It works well both with EAP only and EAP6 + Apache httpd(mod_proxy). Users just run kinit and will be able to be successfully authenticated.
After that, I added the fallback-to-form files/configurations both in the web app and standalone-full.xml. The fallback-to-form works only if httpd stops. If httpd starts, 401 error will always be thrown out.