You cannot use “Monster Reborn”, etc. on “Mazera DeVille” even if he is Special Summoned properly, because he “cannot be Special Summoned, EXCEPT by…”.
If you Special Summon “Mazera DeVille”, his effect activates, and your opponent chains “Mystical Space Typhoon” to destroy “Pandemonium” in a chain, “Mazera Deville”’s effect disappears because his effect needs “Pandemonium” on the field when the effect resolves, not just when it activates (just like “The Agent of Judgment - Saturn”).
If your opponent has less than 3 cards in his hand, he must discard his entire hand.