Thank you for being thorough with your investigation and attempts to remediate this @T3chNoid, it really is appreciated and is more substantive than anything I've seen or heard back from QB support.

I've checked the 3 IP addresses this morning and they look to have been de-listed on SpamCop. There have been a couple of other previous instances where they've come up clean, only to be relisted the next day, so hoping this lasts.