Yes, absolutely. Like det, you could pronounce it the way it ought to by appearance, but it would sound very formal. It's something I would expect at, say, a Nobel prize recognition ceremony.

That's just not English. We say "Most of my..." or "The majority of my...". You can't mix them.

Because "flest av" is simply not correct. There are many situations where you cannot translate word for word between Swedish and English, and this is one.

Intuitively, I would say "de flesta av mina kusiner" but "de flesta kusiner". Just like in English, where you would also say "most of my cousins" but "most cousins".