No Offense, but 12 hour prints are far from unusual. 84 hours certainly are, but even so, they have worked reliably in the past - and they still do with the different print plates.
I understand that you may not trust long prints, but this issue is most certainly linked to the Print Plate, not the duration of the print. That I have to increase the Temps on a Cold Plate for simple PLA up to 60 Degrees in the first place just to get an “acceptable” first layers makes me think that there might actually be a defect or something.
I have messaged the Support, but hopefully I can find another fix.