Is this person originally considered a Muslim, and is it valid for someone like him to lead the prayer, and should he be prevented from residing, or even from coming to the mosque at all, or not?

Calling the Adhan via recording or devices is an innovation (bid'ah). For the Adhan and Iqamah to be valid, Islam, sanity, and male gender are required. As for your colleague's words, they constitute major disbelief (kufr akbar) and apostasy, unless he is afflicted with a mental or psychological illness that prevents him from understanding what he says. In that case, he is not held accountable for his words, but he must be prevented from leading the prayer due to his lack of qualification.