Update!
Boiler was messed up in various ways (leaking, wiring...etc). Navien replaced it underwarranty (phew!) but it still cost around $3.8k to get the system installed and piping fixed/other misc things. HKS PWR was pretty much spot on in his diagnosis. I used Coppersmith plumbing (run by a guy named Jackson). Dude's legit and honest.
But more importantly, here are the pictures.