On a different note...

...I've been reading through the documentation and source code for the libraries you referenced in your code, which I think are these ones:

1. Control Surface
2. Arduino MIDI Library

These already implement the MIDI message parsing using callbacks, e.g.:

• Using callbacks
• Using callbacks .ino

That example stresses the importance of swift non-blocking code. This means that updateFader() should also be non-blocking. You have the option to write your own custom state machine parser or use the libraries' parsers.

Here is how your code could be re-written to use the MIDI library's parser with non-blocking callbacks and non-blocking updateFader(). You may want to double-check which pins you are using for midiSerial because pins 0 and 1 are used for the default Serial monitor or you could use MIDI.sendNoteOn() and MIDI.sendNoteOff().

#include <MIDI.h>
MIDI_CREATE_DEFAULT_INSTANCE();
unsigned int faderMax = 0;
unsigned int faderMin = 0;
unsigned int faderTargetPosition = 0;
void doNoteOn(byte channel, byte note, byte velocity)
{
 midiSerial.write(channel);
 midiSerial.write(note);
 midiSerial.write(velocity);
 faderTargetPosition = velocity * INCOMING_VELOCITY_SCALE;
 // Clip if out of bounds.
 if (faderTargetPosition > faderMax)
 {
 faderTargetPosition = faderMax;
 }
 else if (faderTargetPosition < faderMin)
 {
 faderTargetPosition = faderMin;
 }
}
void doNoteOff(byte channel, byte note, byte velocity)
{
 midiSerial.write(channel);
 midiSerial.write(note);
 midiSerial.write(velocity);
}
void setup()
{
 MIDI.setHandleNoteOn(doNoteOn);
 MIDI.setHandleNoteOff(doNoteOff);
 MIDI.begin(MIDI_CHANNEL_OMNI);
 . . .
}
void loop()
{
 // Parse the MIDI messages and call the callbacks.
 MIDI.read();
 if ((millis() - lastDebounceTime) > debounceDelay)
 {
 int totalCp = touch.capacitiveSensor(30);
 int faderPos = analogRead(FADER_PIN);
 int faderHiResMIDI = faderPos * 16.0146627566 - 8191.5;
 if (totalCp <= minimumCp)
 {
 // Not touching fader.
 //if (incomingCommand == ch1Vol)
 //{
 updateFader(faderTargetPosition);
 //}
 }
 else
 {
 if (faderPos == lastfaderValue)
 {
 // Touching fader.
 MIDI.sendNoteOn(Db_1, fullVel, ch_1);
 digitalWrite(MOTOR_DOWN_PIN, LOW);
 digitalWrite(MOTOR_UP_PIN, LOW);
 }
 else if ((faderPos > lastfaderValue + 1) or (faderPos < lastfaderValue - 1))
 {
 // Moving fader.
 MIDI.sendPitchBend(faderHiResMIDI, ch_1);
 digitalWrite(MOTOR_DOWN_PIN, LOW);
 digitalWrite(MOTOR_UP_PIN, LOW);
 }
 }
 lastfaderValue = faderPos;
 lastDebounceTime += debounceDelay; // Constant interval.
 }
}
void updateFader(const unsigned int& faderTargetPosition)
{
 const unsigned int currentPosition = analogRead(FADER_PIN);
 if (faderTargetPosition < currentPosition - tolerance) // Below the tolerance band.
 {
 digitalWrite(MOTOR_UP_PIN, LOW); // Switch the up control off before switching the down control on.
 digitalWrite(MOTOR_DOWN_PIN, HIGH);
 }
 else if (faderTargetPosition > currentPosition + tolerance) // Above the tolerance band.
 {
 digitalWrite(MOTOR_DOWN_PIN, LOW); // Switch the down control off before switching the up control on.
 digitalWrite(MOTOR_UP_PIN, HIGH);
 }
 else // Within the tolerance band.
 {
 digitalWrite(MOTOR_DOWN_PIN, LOW);
 digitalWrite(MOTOR_UP_PIN, LOW);
 }
}

void calibrateFader()
{
 const byte num_samples = 10;
 digitalWrite(motorUp, HIGH);
 analogWrite(motorPWM, motorSpeed);
 delay(300);
 digitalWrite(motorUp, LOW);
 unsigned int faderTotal = 0;
  for (byte i = 0; i < num_samples; i++)
 {
 faderTotal += analogRead(wiper);
 }
 faderMin = faderTotal / num_samples;
 Serial.println(faderMin);
 digitalWrite(motorDown, HIGH);
 analogWrite(motorPWM, motorSpeed);
 delay(300);
 digitalWrite(motorDown, LOW);
 faderTotal = 0;
 for (byte i = 0; i < num_samples; i++)
 {
 faderTotal += analogRead(wiper);
 }
 faderMax = faderTotal / num_samples;
 Serial.println(faderMax);
}

void calibrateFader()
{
 const byte NUM_SAMPLES = 10;
 digitalWrite(motorUp, HIGH);
 analogWrite(motorPWM, motorSpeed);
 delay(300);
 digitalWrite(motorUp, LOW);
 faderMax = AnalogueAverage(wiper, NUM_SAMPLES);
 Serial.println(faderMax);
 digitalWrite(motorDown, HIGH);
 analogWrite(motorPWM, motorSpeed);
 delay(300);
 digitalWrite(motorDown, LOW);
 faderMin = AnalogueAverage(wiper, NUM_SAMPLES);
 Serial.println(faderMin);
}
unsigned int AnalogueAverage(const byte pin, const byte num_samples)
{
 const byte ROUND_OFF = num_samples / 2;
 unsigned long total = 0;
 for (byte i = 0; i < num_samples; i++)
 {
 total += analogRead(pin);
 }
 return (total + ROUND_OFF) / num_samples;
}

Fourthly,faderMin and faderMax should be just that – the minimum and maximum extent of fader travel. But by adding/substracting tolerance from min/max it is unnecessarily reducing the range. The tolerance is effectively being doubled by adjusting for it in bothsetup()andadjustFader(). The extent of fader travel can be obtained by taking an average of several values in calibrateFader(). Then the tolerance can be adjusted for only once in adjustFader() as described above.

void calibrateFader()
{
 const byte num_samples = 10;
 digitalWrite(motorUp, HIGH);
 analogWrite(motorPWM, motorSpeed);
 delay(300);
 digitalWrite(motorUp, LOW);
 unsigned int faderTotal = 0;
 for (byte i = 0; i < num_samples; i++)
 {
 faderTotal += analogRead(wiper);
 }
 faderMin = faderTotal / num_samples;
 Serial.println(faderMin);
 digitalWrite(motorDown, HIGH);
 analogWrite(motorPWM, motorSpeed);
 delay(300);
 digitalWrite(motorDown, LOW);
 faderTotal = 0;
 for (byte i = 0; i < num_samples; i++)
 {
 faderTotal += analogRead(wiper);
 }
 faderMax = faderTotal / num_samples;
 Serial.println(faderMax);
}