Как насчет использования регулярных выражений?
var pattern = /(,\s)?Episode\s\d+((,\s)?)/g;
var guests = data[x].name.replace(pattern, "");
Контрольные примеры:
var str1 = "Kevin Young, Antoinette Robertson, Gen. Michael Hayden, David Blaine, Episode 63, Episode 64, Episode 65";
var str2 = "Episode 63, Episode 64, Episode 65, Kevin Young, Antoinette Robertson, Gen. Michael Hayden, David Blaine";
var str3 = "Kevin Young, Antoinette Robertson, Gen. Michael Hayden, David Blaine";
var str4 = "Episode 63, Episode 64, Antoinette Robertson, Episode 65";
var str5 = "Episode 63, Episode 64, Episode 65";
var str6 = "Episode 63";