As for Harold, he was never a feral ghoul. Also Bethesda made him a "Special FEV Mutant" and not a ghoul. Why is he in Fallout 3? Cause Bethesda wanted to have him in the game as fan services and completely [censored] things up
Harold has always been a mutant. Even in Fallout 1. He even has specific dialogue stating he is a mutant Fallout 1. Bethesda didn't make him anything but a tree in the ground.
http://www.youtube.com/watch?v=GCK_FV27SN8 Harold talks about turning into a mutant at Mariposa around 3:50 ish.
Harold is actually a normal mutant Changed by an FEV strain from before the master restarted experimenting with FEV and creating super mutants. Super mutants actually didn't start appearing until the master came along. Someone in Fallout one, when talking about super mutants, says something akin to "These aren't like regular mutants, they're hulking monsters."
So in actuality Super mutants are really the "Special FEV mutants."
As for jet, even if Myron dies in every ending I'm sure the Mordinos has something lined up to keep the jet flowing, and New Vegas alludes to this since jet is still heavily used in the Reno area in 2281. obviously that doesnt explain why it is in the CW but i like to think that bands of merchant/scientists/communities looking to establish themselves in a new wasteland followed the path that the Lyons BoS carved east. Following like wild dogs followed armies marching in medieval times, picking up the scraps left in the lyons wake and finding semi shelter from larger dangers of the wasteland. It could have been these types that brought jet to the Capitol wasteland.
Now why it is in 200 year old cabinets where no one has ever been can only be explained as kinda [censored] stupid.
And as for cutting jet, just put a 30% oxygen 70% jet mixture and boom it's cut. Since it's a gas the thing you would cut it with is the thing you breathe in all the time. Im not going to get deep into the real intricacies of drugs so as to not break ToS, but to cut a drug/medication you just mix the drug with something that costs almost nothing and is of the same color and consistency (this process is also done with legal drugs, a portion of it being an inert placebo and the rest being the medication. Since air fits that criteria for pretty much any gas it's the most logical thing to cut jet with.