I completed his quest wherein he is exiled from the Brotherhood, but I still seem to have lost the ability to use him as a companion after I sided with the Railroad. Does anyone know if this is an error or if he is still non-recruitable even though he is no longer tied to the Brotherhood?

If you truly can't side with the Railroad and have Danse as a companion, does anyone know if you can side with the Minutemen and have him as one?

If you go against the Brotherhood in any way, you will lose Danse as a companion. Despite his exile, he will always support Maxson and his friends in the BoS.