Earlier someone posted on the pronunciation of a few Japanese designers and I just wanted to add a few additions to what was mentioned:
Rei Kawakubo-
Rei: the "r" is like a Spanish rolled r (as someone mentioned), but roll it only once. Don't go with a full rolled "r." So, you get something sounding closer to "Day" than "Ray" or "Lay" or the "r" in Roberto.
Kawakubo- there is actually very little stress in Japanese, compared to English. But, there is a SLIGHT stress on the second syllable (as someone mentioned), but not too much. So, it is more like KAH- WAH- KOO- BOW than kaWAAAAkubo.
Issey Miyake-
EE-SAY ME-YAH-KEH The last syllable ISN'T like "key", but more like "keh."
Mihara Yasuhiro- MEE-HAH-RAH (again, the "r" is between an English r, l, and d... the same as in Rei Kawakubo) and YAH-SUE-HEE-ROW, with a SLIGHT stress on the 3rd syllable.
As for the other questions, Martin Margiela is Mar-tan (LIGHT on the "n" though, not a full American 'n") and Mar-zhee-ela. Also, go easy on the "zhee."
John