Thanks Tef8, chat worked better, insofar as I was told the "negative settled cash" was forward looking as of today 9/21. Beyond that, I've had to deduce the trouble myself.
So, I reviewed my statements for 9/18 and found positive day-end cash balances, but they were very small, under $10.
Perhaps subsequent price allocation or commission adjustments were involved, but by mid-day Tuesday, IB's calculation for settled-cash for Monday's trades seems to have swung a few dollars negative and shut down my ability to make purchases then and there -- which I was in the midst of doing via order allocation.
Instead of blocking the allocated portions, IBs allocation process sent shares anywhere and elsewhere where there was buying power, which was a can of worms.
Anyway, I've no complete, clear response from IB, nor evidence of a resolution, but my best guess is: in their margin IRA, don't cut the day end cash balance too close. Subsequent adjustments could more than wipe it out and shut everything down without explanation.