Driving the IRLB8743's directly out of the buffers is fine as long as the buffers put out 5V. This also removes the inverted heater lines (no need for FETs to invert them, and their associated components), and means all the ESTOP pull-up's all become pull-downs.
The only issue I see is that with all the specs I've seen on all HC/AC/HCT/ACT (wrong, see below), logic "High" is considered anything from 0.5x (typical) to 0.7x VCC (the guaranteed limit at the temp specs of the package - -40C to +85C). At 5V, that means between 2.5v (typ) <-> 3.5v (max). This means as long as we're within the typical window (which never seems to be 100% defined on these things) we should be OK.
To test this in-circuit without redoing the board, we can probably just hack an existing board to get the same arrangement. Just lift the VCC pin for the buffers and power them directly from 5V (possibly take it from +VGATE if you install a 7805 in place of the 7812, since we won't need the 7812 any more), remove the excess FET components and pullups and either short the Gate-Drain on one of the driver FETs, or replace it with (say) a 10 ohm resistor to limit the inrush current to the gate.
For example:
For D10 (heated bed, the one that needs to be driven the hardest since it handles the most current), remove Q204, R214 and RP1. Remove the 7812 and replace it with a 7805. Lift pin 14 on U1 (74HC125, or replace it with a 74ACT125 but keep Pin 14 lifted) and run it to +VGate. Put a 10 Ohm resistor between the Gate and Drain pins on the pads for Q204, or just bridge them with a piece of wire. You can also add in an ESTOP pull-down resistor between the Gate and Source pins of Q204 as well (eg: 10K).
This would at least allow us to test this works 100% before we go ahead and get boards made.
Note: The output LED won't light up anywhere near as well (if at all), so you probably want to replace R220 with something smaller if you want that to work.
The only issue I see is that with all the specs I've seen on all HC/AC
To test this in-circuit without redoing the board, we can probably just hack an existing board to get the same arrangement. Just lift the VCC pin for the buffers and power them directly from 5V (possibly take it from +VGATE if you install a 7805 in place of the 7812, since we won't need the 7812 any more), remove the excess FET components and pullups and either short the Gate-Drain on one of the driver FETs, or replace it with (say) a 10 ohm resistor to limit the inrush current to the gate.
For example:
For D10 (heated bed, the one that needs to be driven the hardest since it handles the most current), remove Q204, R214 and RP1. Remove the 7812 and replace it with a 7805. Lift pin 14 on U1 (74HC125, or replace it with a 74ACT125 but keep Pin 14 lifted) and run it to +VGate. Put a 10 Ohm resistor between the Gate and Drain pins on the pads for Q204, or just bridge them with a piece of wire. You can also add in an ESTOP pull-down resistor between the Gate and Source pins of Q204 as well (eg: 10K).
This would at least allow us to test this works 100% before we go ahead and get boards made.
Note: The output LED won't light up anywhere near as well (if at all), so you probably want to replace R220 with something smaller if you want that to work.