Neither Twelfth nor Fifth have a silent f. If you don't pronounce it, it's because you're pronouncing it incorrectly. Of course if you pronounce twelfth correctly you may get a cramp in your tongue, but that's neither here nor there.
Now, Kamard gave good descriptions of the pronunciations of the names, though I pronounce a couple of them differently, none of his looked glaringly wrong.
These are the two I pronounce differently, caps represent the accent is on that vowel.
Evard - EH-vard
Mordenkainen - MORE-den-KANE-en
So... there you have it, names of wizards as typical americans would pronounce them. Is that the correct way? Who knows? Only the people who came up with the names can actually say what is correct and what is not.
-The Souljourner