Forgot to mention in the interim, my 1644 went away to be replaced by a 1438 (if memory serves). Scott suggested a bad MDM (contained within the dc-dc converter.)
Thus, I replaced the dc-dc converter with a good used unit that I had scavenged from a wreck in Pa. two years ago. IMA function restored. No IMA light, no codes, no CEL. Recharge, assist and autostop all functioning as normal.
I’m chalking this one up as a win, but replacement was a tedious, time-consuming procedure in a tight space. Luckily, I am small enough to stand in the spare tire well and the dc-dc converter isn’t nearly as heavy as a high voltage pack.