With regard to the spark, VE, and other tables in the $8d mask: where do the cell boundaries (RPM vs. VE/MAP) fall relative to the data values? I understand how the BLM cell boundaries work, but have found no information about the tables. What I need to know is, for instance, in the 30 MAP, 2000 RPM cell in the spark table, does that cell include rpm from 2000 to 2200 rpm (indicating lower boundary), from 1800 to 2000 rpm (indicating upper boundary), or from 1900 to 2100 rpm (indicating range center)? I've searched on "8d", "cells", "threshold", as well as a number of other terms, as well as studied the ANHT hack, all to no avail.

Here is a thread with some good links. There is also one by Rbob.
Don't recall if it's linked there or in the stickies. But it's a little easier to understand.
Basically the listed value for ea row/col is the lower bound with the next higher value being the upper. There is also a small amount of hysteresis built into the code also.
For RPM/25 for instance. The same VE will be used up to +- 12.5 rpm IIRC due to rounding. Not enough to really matter.

You might try a search on 2d & 3d lookups.