Batron 24003V LCD-->PCF8754T Backpack-->Mega 2650 r3 ...I2C Device Not Found

I have a Batron 24003V 40x2 LCD display, attached with an FFC ribbon cable to a FFC–>1x16 circuit board board, soldered to a PCF8574T Backpack, connected to my Mega 2560 r3’s I2C pins. I couldn’t get any I2C LCD test code to work, so I ran several different I2C bus scanners, all of which told me there were no I2C devices to be found. The backlight lights and the contrast control on the backpack adjusts the LCD fine so I think I have it wired correctly. I’ve tried:

-Switching SDA and SDC to pins 20 and 21 on the Mega
-Several different Liquidcrystal_I2C libraries
-Multiple (pretty much all the standard demo) sketches.
-Running the backpack from 3.3V (specs said it’s good up to 7V but I thought it wouldn’t hurt to try)
-Using external power
-Checked/reflowed solder on my own work…and also on the backpack 1x16 pins
-Checked the ribbon cable
-Checked continuity in my jumpers
-Changing the I2C address within the default range on the PCF’s datasheet
-And finally, I read that the pullup resistors on the backpack might need to be removed, if the circuit already has pullup resistors (am I understanding correctly that the Mega has pullup resistors on the chip?)…so my last desparate measure was desoldering the pullup resistors from the backpack.

The vendor mentioned that the display uses an NT3881 chipset, supposedly HD44780 compatible.

Of course, in my infinite wisdom, I soldered the backpack directly to the adapter board instead of using a socketed connector, so testing the display directly, which was my next logical step, is going to be quite an undertaking without damaging the backpack. :-[

Some pictures so this will make more sense:

Update: After pouring over the Texas Instruments Application Report on calculating i2c pullup resistor values, I experimented using parallel (to the internal pullups) resistors making the total pullup resistance 960 Ohms, 1.38K, and 3.2K. Still no joy–yanked the backpack and will double-check that the display itself works next.