Long awaited update:
Backstory- I did this repair in my parents garage as I didn't have the room & I though it'd be handy to have my folks around to hold a wrench from time to time (they're both retired) & since they have a two car garage they don't really use, why not?
Diagnosed- After I put everything together the car ran for 2-3 minutes & died, then would not restart. Though very confident in my work, I double/triple/quadruple checked everything & I still couldn't get it to run for more than 20-30 seconds. Eventually, I broke down & took it to the dealer. After spending "Much longer than we should have" diagnosing it, they found the car to have a clogged catalytic. Strange, how could the car have gone from running great, to clogged catalytic because a spark plug blew out....... I towed the thing home as a factory catalytic costs $1200...
Cause- Removing the catalytic should've been easy but thanks to salty roads I had to remove the entire exhaust & grind off each of the cat's bolts to separate it from the rest of the system. Borescope of the pipe found the culprit- WHILE THE ENGINE WAS APART I SARAN WRAPPED THE TOP OF THE MOTOR TO KEEP STUFF OUT BUT THE EXHAUST WAS LEFT OPEN. TURNS OUT A MOUSE WENT DOWN THE PIPE & MADE A COZY LITTLE HOME FOR HIMSELF. The 2-3 minutes of initial running was probably what it took to push the entirety of the mouse nest & baby mice flush against my catalytic to clog it.
All fixed- To get the many small holes of the catalytic cleared would have been difficult & the thing was kind of a rusty mess anyway so I purchased a brand new MagnaFlow Catalytic Converter (Part #51768) and my Fit finally has a clean bill of health.

Thanks all for the help, who could've guessed it was all because of a mouse........