A quick follow up: Almost two years later, Heck is now a sweetie.
It took months of resisting making eye contact. Every morning I'd spend fifteen minutes letting her see me play with Heaven in front of her. After a couple months, she began to inch down the cat tower and take tentative swipes when the toy on a string would pass near her. Eventually she got brave enough to hop down from the tower to pounce the toy before rushing back up the tower to hide again. A few more months, and she now comes down and plays freely and doesn't mind eye contact.
After about a full year, she began to accept being picked up for petting and brushing. After enough of that, she decided I wasn't a threat as long as I'm sitting at my desk. She even jumps up on the desk on her own if she needs some attention. Coworkers have gotten used to hearing her purring through my headset microphone during conference calls. She seems to have figured out that when I put the headset on for a meeting, my hands are about to be free for petting.
Previously, I had put the cat food in my office. Now I put it in the kitchen so Heck has to venture out of the room in order to eat. I have to be sitting in the kitchen in order for her to come out and eat, but it's an improvement. Previously she wouldn't venture out of the office at all while I was awake.
Almost every morning for the past few weeks, she now brings me a toy mouse when I go into my office after breakfast. She makes soft chirping mews until I brush her out and give her a few treats, which is incredibly cute.
Both cats still scatter whenever I stand up. They're terrified of being on the ground when I'm standing. But they join me when I'm sitting at the computer or on the living room couch. And when Heck scatters, if it's evening she goes up the cat tower to the post nearest the door. She's figured out that she can usually get a few extra caresses from me when I pass by on my way to bed, which always elicits a strong purring.