I've put the conversions on hold for a little while since my installs after the first successful one didn't work no matter what I tried. Cleaned up my soldering profiles, ensured my alignment was dead on, sought help from very knowledgeable people including N64 himself, to no avail. At some point I stopped counting, but I believe I registered 9 or 10 failures and decided to take a step back, since the time wasted for no results became a source of frustration.
Uploaded some pictures here: https://imgur.com/a/z0YPuoN
Ultimately I decided the sensible continuation would be to employ an x-ray device to verify correct soldering of the interposer. I found a dental device that I could accommodate with my budget and the device has since arrived. The sensor area is small (21x21mm) but resulting images are surprisingly usable. Can't tell for sure if this will allow detection of voids or head-in-pillow easily, but certainly I can identify spots where the balls floated slightly. I will adjust the interposer reflow profile to remain above liquidus slightly longer. More updates soon!