Diatonic is a good mode for straight-up 2nd guitar harmonies, as described in the reply above.  Whether or not you use both Pitch A & Pitch B (for any combination of 3rds and unison) depends on the textural 'thickness' that you want.  The micro-variations in the pitch shifters can give you a nice double-tracked sound against your dry input.  With an expression pedal controlling the dry/wet mix, it's possible to fade the harmony line in & out very smoothly .

For a phat, double / triple-tracked texture, I like to use the Quadravox mode.  There's a nice spread across space & time that shines through a stereo rig, with harmony & unison at left, right, and center.

Mix –  50%

Pitch Mix – centered

Pitch A  –  A: +3rd

Pitch B  –  B: Unison

Delay A  –   D:  4 ms.

Delay B  –  .  A . B . C . D   (approx. 1 o'clock)

Key/Depth –  tonic / root note

Scale/Speed –  Maj or min or …

Xnob –  C: Unison

Ynob –  D: +3rd

If you set an expression pedal to vary between 0-30 ms. on Delay A, it will vary the timing enough to enhance the ensemble feel, with some nice crossfading between short delay times.  Anything longer than that, and the 'ping-ponging' of voices across the stereo soundstage really pops out.

---

I was originally confused by the terminology in your second question.  If you had needed 3.5 semitones down (tuned down between 'B' and ''C'), that added quartertone would've locked you into using the H910/H949 mode.

Mix – 100%

Pitch Mix –  fully CCW (left)

Pitch A  –  A: 0.818 – A: 0.824

Pitch B –   n/a

Delay A –   n/a

Delay B –   n/a

Key/Depth –  Modern

Scale/Speed –  Normal

Xnob –  n/a

Ynob –  n/a

More likely, you need that  A-D-G-C-E-A mentioned.  You could use the H910/H949, Harmodulator, or even the PitchFlex mode for this  Concentrate on the same controls as the preset listing above, with 100% wet and a few key changes to the settings.  In the H910/H949 mode, you might change the Scale/Speed to Chromatic and Pitch A to  A: 0.667.

In the Harmodulator or PitchFlex modes …

Mix – 100%

Pitch Mix –  fully CCW (left)

Pitch A  –  A: – P5

… with all of the other knobs at neutral or zeroed settings.  Becoming a 'DropTune'-type device really isn't one of the Pitchfactor's strong points, but you can squeeze out some interesting and usable tones; even when you bypass those extensive modulation capabilities.