If my Whoniverse game never takes off, I just might try a Mortal Horror game. Here are my answers to the questions:
1: Age. Would it need to be young adult to be captivating? Could you do "age spheres" such as High School, College, Adult and run plots for each?
I'd base it very loosely on the comic book Locke and Key by Joe Hill (Stephen King's son). I'd call it Welcome to Lovecraft and set it in modern-day Lovecraft, MA, the fictional town from the comics, but would not use (much of) the story or characters. Lovecraft is a strange town where the supernatural and unusual happens but no one talks about it. Not the adults, anyway. They always forget. I'd let PCs be students at Lovecraft High and Miskatonic University, or those few adults who can, for various reasons, notice and remember. While the HP Lovecraft influences are obvious, it wouldn't just be that. I'd bring in lots of horror movie stuff like Camp Crystal Lake, vampires and werewolves and sea creatures and zombies and... the whole toybox. The idea would be that Shit Is Not Right, but no one believes it.
2: How often would plots need to be run? Let's say, for gits and shiggles, that I'm running a game and am also the only staffer running plots. I could only do so probably once a week, on Monday evenings. Would that be enough? Twice a week? Three times?
I could/would run at least a scene a week, sure.
3: Player Run Plots. I'm a HUGE fan of PrPs. What tools would players want/need to run their own plots?
I'd let people run with it. Borrow from comics, movies, TV... have fun. Anything can happen in Lovecraft.
4: Mortality. Should death be a very real possibility? Should players go into character generation knowing their PC could be short-lived? Should there be a "victim" opt-out option for players who don't necessarily want to have their characters killed (which may make them ineligible for some scenes)?
I'd let people opt out, but if they want in on the nastiest stuff, death can happen.
5: The monsters. Strictly staff run? Should players be able to take on the role of "Suburban Vampire" for the length of a plot knowing that, in the end, he will be slain?
As I said in #3, go have fun.
6: System. What system would work for this? I think WoD or the current edition of Call of Cthulhu would work fantastically.
I'd be open on system. Not WoD, though. Something that doesn't have 30+ books and is less about fistfuls of dice and XP and number-crunching and more about drama and story. FATE, etc.