I finally got my car back together and can report back. Thank you to everyone that tried to help diagnose the issue. It idles as expected now.
With the engine out, I found the problem was most likely this pipe (930-110-328-05) circled in the image below, which our host has available for a mere $237.00. You can see my shiny new one installed (motor is in process of being refitted in photo).
Mine was both swollen at the hose, and cracked in half where the rubber hose meets the compression fitting. It was very hard to see/diagnose behind the heater blower (before I removed the blower for my heat and exhaust backdate). I discovered how bad it was once I had the engine out. I did not find anything else that I thought was bad enough to have caused the issue.
Thank you to everyone who tried to help diagnose the problem, and for those of you with a similar issue, perhaps this will be a good lead.