on the first one, according to http://harrypotter.wikia.com/wiki/Dementor

After Apparating to Hogsmeade, Harry, Ron, and Hermione set off a Caterwauling Charm and hid under Harry's Invisibility Cloak. Unable to find them, the Death Eaters dispatched Dementors to attack the trio, and Harry was forced to cast his Patronus to protect them from being Kissed.

My original point about patronuses turned out to be more hair-splitting than anything else...but it also turns out that Rowling's take on the partronus totally diverges in lesser-cannon.

"Wonderbook: Book of Spells is the closest a Muggle can come to a real spellbook. I've loved working with Sony's creative team to bring my spells, and some of the history behind them, to life. This is an extrodinary device that offers a reading experience like no other." —J.K. Rowling on Wonderbook: Book of Spells -giving us a source of Rowling-approved info...

In that book, a guy called Raczidian tries to cast a Patronus. He produces, instead, a swarm of maggots....Which turn and eat him up.

MoR "dark-wizards" simply find that the spell fizzles.