I currently have the middle as the heated bed thermistor and the bottom is the hot-end thermistor. Is this correct?
yes, this is correct
I have 120V 750W, KEENOVO Universal Flexible Silicone Heater to a SSR with an outlet switch. I used Walter’s thread but does not show where the controller connections are. (MKS GEN 1.4 board). Currently have connections to one that is closest the power connects on MKS board. Is this correct?
Someone else will have to answer this one...
I saw that someone had wires coming out from the next set of connectors down. Currently my fan for the Hot-end is connected to the power supply. I believe that is for fans. If is for fans, what fan gets hooked up here?
Not sure what connectors you are referring to?
Do you upgrade the firmware with the Printer with the AC power off?
AC power should be OFF
When using Arduino IDE, what selection is used for the board? (Arduino Mega?)
Board: Arduino/Genuine Mega or Mega 2560
Processor: AT Mega 2560 (Mega 2560)
Can the manual be updated to go get the USB VCP Drivers (FTDI drivers) if the FT232 USB to UART do not load? (I remembered this with LED controllers that I had to flash the chips.)
(above my paygrade, lol)
When performing the calibrations using Arduino IDE, is the AC power on?
You do not use the Arduino IDE for calibration - it is only for flashing the firmware...and AC power should be off for that.
When does the power get turned on?
After you have flashed the firmware to the board and you are ready for testing.
Should the belts be off to check motors turn in correct rotation?
Not necessary... with ALL power off, move the carriage assembly to the middle of the machine, then power up and test movements in small increments to make sure things move in the correct direction.
Do all changes with MKS GEN 1.4 get re-compiled through Arduino IDE and reloaded?
In short, yes... any changes you make to the firmware must be flashed to the board.
Is Lexan Electronics case a good idea?
it is fine.