Should an Avel who is Sephardi say a Sephardi kaddish if he is in a shul which is a different Nusach? And should a Nusach Ashkenaz (or Nusach Sephard) who is saying kaddish in a Sephardi Bet Knesset say his own Nusach or not?

Kaddish should be said according to the Nusach of the Shul, not according to the Avel's personal Nusach. [אג"מ או"ח ח"ב סימן כג וח"ד סימן לד, וכ"ד הגרש"ז אויערבאך זצ"ל בהליכות שלמה - תפלה פרק ה סעיף כ דבר הלכה ס"ק לג]