i would imagine there is nothing to tell.
krister is more than qualified and had all the right equipment...
if it were me, i would remove the board and "reflow" /rework the board in an oven.
this is common practice for laptop repair (several years ago, everyone was investing in these ovens because there were MILLIONS of bad laptops with nvidia chipsets
i have not reread the entire thread, krister may have tried the reflow approach
i assume a replacement system board is prohibitively expensive. sometimes you can't beat an imaginative cheat, such as placing a small heater in position to keep the board at the temperature it prefers.
as these printers go from off or idle to printing state, components heat up and the board flexes, causing intermittent problems. they can be very difficult to troubleshoot & the solution may be different, every time.