Is it permissible to go out with women who do not wear the niqab, even though they observe complete hijab, taking into account that they do not believe it is obligatory, while I believe it is obligatory? And is there any sin in that?

If the sisters wear a hijab that covers the head and the rest of the body, except for the face and hands, then they are upon great goodness. Covering the face and hands is a matter of scholarly disagreement based on ijtihad. Although the opinion favoring the obligation to cover them is preponderant, the matter is broad, and there is no harm in associating with those who do not cover them, following those who do not consider it obligatory.