Can donor hair be taken from the back of the neck, below the collar line? (photo)

In general most people have about 8000 grafts available for harvesting without making the donor area look depleted.This applies mostly to the Strip (FUT) procedure.For FUE 4000 to 5000 grafts may make your donor area look very depleted if you keep you hair short.

You had a large number of grafts taken and probably still have some scabbing in the donor area. Small scabs in the donor hair can force the hairs apart slightly and slight change the direction of the hairs which makes it look unusual compared to before surgery. It's difficult to tell...

It may take several weeks to months before the sensation on your scalp returns to normal.  It is different for every person.

It is normal to shed transplanted hairs weeks after surgery.  The new hairs may take 6 to 12 months to grow.

Single hairs for the face are generally preferred over multi unit follicles. 600 is very small and should not create any obvious deformity.

You just had surgery.  It would be best to wait until you see your results before considering another surgery.  You can plan future surgery with your surgeon.