1. Who is your favourite Death Eater? Why?
They're all kind of loathsome toadies. They suffer from the same problem as the Sith. Either they're not strong enough to be really powerful, in which case they are simpering lackeys, or they are powerful enough to be really strong, which case they become the master and aren't a padawan or death eater any more. They are the force for evil. So there's your answer. I don't like any of them.
2. Do you have any speculation regarding the relationship between Voldemort and Bellatrix? Nope. He's the master, she's a drooling fangirl. Although there are cult leaders and other loathsome vermin who take unfair advantage of their female followers, Voldemort doesn't strike me as that type. He really is consumed with other things.
3. How or why do you think Peter Pettigrew was made Secret Keeper for the Potters at the last minute, a week before the Potters' death? Beyond what Sirius explained? It's hard to believe that the Potters ever thought it was a good idea, or why Lupin would be more suspect than Pettigrew. Safest of all would have been to make Dumbledore the secret keeper. Voldemort never attacked him or his directly. So, why not? If you were going to switch off Black for reasons of Black being a) too obvious and d) too accessible. I think Pettigrew was a plot device.
4. Which Death Eater would be most likely to try and overthrow Voldemort and become the leader? (Or is it possible they are all so loyal that this could never happen?) Malfoy the elder. He wants power. And he had it through money and influence. He was probably least pleased when Voldemort returned because he lost the most and had the most to lose. Unfortunately he lacks the drive and real creativity to be truly powerful. He's had things too easy his whole life.
5. Hagrid once said "When a wizard goes over ter the Dark Side, there's nothin' and no one that matters to 'em anymore..." Think about this statement in terms of Snape. Does this apply to him as well? Did something or someone matter enough to him to become a spy for the Order?
I'm not convinced he wasn't playing a double game yet. I haven't decided where his allegiance is yet. But yes, I suspect he's the only thing that matters to him.