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I think we are getting close.
1, 5V are made by Arduino Mega. Try to measure 5V on Arduino alone (without Ramps on it). If it will be around 5V Arduino is OK and something on your Ramps is shorted. Than try to measure 5V on Arduino with Ramps but without drivers. If it will drop to your 4.3V, check Ramps for some leftover solder, small balls of solder, unsoldered connections, and wash away residual flux with toluen. If Ramps don't cause the drop, than check drivers one by one.
2, If there is less than 4.5V, the ATMEGA2560 is undervolted and may do some unpredictable errors The processor may work with lower voltage, but it also have to have lower clock speed. See page 22: http://ww1.microchip.com/downloads/en/devicedoc/atmel-2549-8-bit-avr-microcontroller-atmega640-1280-1281-2560-2561_summary.pdf Arduino have to be manufactured as such (you cannot change clock speed easily).
3, For microstepping settings of DRV8825 see this picture: http://doku.radds.org/wp-content/uploads/2015/02/MicroStepping_Modes_tbl.png
(The pins are not "shorted". Either the MS0-2 are open=OFF or connected with 5V = ON). For Z asis you don't need microstepping at all so you can do fullstep = 1 or halfstep = 1/2. For other axis (X, Y, E) = 1/8 or 1/16 is sufficient. You have to change the DEFAULT_AXIS_STEPS_PER_UNIT settings in firmware accordingly (also use correct movement speeds in Cura or Pronteface - beware, these are typically in mm/min) otherwise the motor may try to move too quickly and stall.
4, This was not on my video clearly explained - The Dir, Step and Enable pins of the drives are connected do ATMEGA data pins. To set them HIGH voltage between 2.2-5.2V have to be used (in my case 4.2V). To set them LOW less than 0.7V have to be measured on these pins. To make the motors move Enable have to be LOW and Reset have to be HIGH (reason why it is connected straight to 5V). Dir depends on the movement direction and Step gives short pulses that can be checked only by oscilloscope.
No problem with late response, I am quite busy too and the video took me a while either ;)