Trim hair under 1 mm, several months after FUE: will white dots be visible?

If your hair length is under 1mm, your FUE scar will likely be visible. FUE is not a scar-less surgery.This is a misconceptiop.

yes , it is true that you may have pin point scars on donor site but, they would be invisible above length of 1-2 mm length of hair. Beside that scar formation may change from person to person .Some of may have more visible, white , little bumping hypertrophic pin points and some of people would have almost no visible scar .I think you would better to use hair long efter surgery to cover morearea and to have a better result.But you can not calculate number and sessions of hair restoration , it would better to consult a hair restoration surgeon .

You  will have punctate scars after FUE and they will be visible. Shaving the head after FUE is not an option as the scars will be visible.

Small punctate scars always appear after FUE. The degree of scarring reflects (1) the size of the punches used and (2) your healing.  The more FUE you have, the more the scarring and the more evident will be the scarring as the donor area density will be decreased calling attention to them. The way you treat this is with Scalp Micropigmentation.  see web reference below