Hi pjmelect;
You're right, it's a really interesting problem. I played a bit with your above project and found the issue what you have written. After lots of NOP were added (see new asm file), was finally eliminated the error (even with two new analog resistance, even the clock is also 8 MHz). Run the new DSN and see the Watch window, the newly added error counters do not increasing (remain zero). Do not ask me why and how, but I'm sure that the "Simulator Options" menu should / could change something. The reason is otherwise known, it is called RMW (Read-Modify-Write) problem of the PIC16 non-enhanced family (no LATx register).
Sorry, no more time, but I hope this also will help you.
zuisti