SA OC 30:2 rules that
One is prohibited to put Tefillin on at night since he may forget
[that he has them on] and sleep in them. But if he puts them on before
the sun sets and it happens to get dark, even if he wears them the
whole night, it is permitted.
The Rambam (Hilchot Tefillin end of 4:11) rules this is a transgression d'oraita.
A person ...
Rav Chaim Kanievsky was asked this in Daas Noteh pg.298. He was asked if one needs to be makpid to davka leave out one hair and not more since the Shulchan Aruch uses the language of hair and not hairs. Rav Chaim answered there is no kapeida,and one can do more and in fact the minhag is for the Sofrim to put more out. So from the strict reading of the Zohar ...
I believe they typically have multiple Tefillin bags at their booths for different head sizes and they are quite good at adjusting the knot on the shel rosh if needed. I believe they put Tefillin on people based on the Shulchan Aruch Harav and am not aware of any heter kiruv on this point that they hold by.