Margiela is pronounced in 4 syllables: Mar-jhee-ell-a. I know a number of people who've worked for- and with him, and they all pronounce it that way, so I assume it's correct.
Oh, and yes, you pronounce the "z" at the end of Hermes.
Oh, and yes, you pronounce the "z" at the end of Hermes.

), and then they ask me to repeat it several times until apparently they get too embarrassed to ask again
Perhaps if I pronounced it Loo-bootin (to rhyme with boot scootin')??

You're right, it is. Margiela is originally from Genk, Belgium. A large percentage of Genk residents are of Italian descent, as during the first half of the 20th century, many Italians immigrated to the region in order to work in the then-thriving coal-mining industry. Just a little history...
unsure: