It is only necessary to negate something that is possible, not something that is impossible. Thus, when G‑d said “My face may not be seen,” He meant that the workings of Divine providence can be perceived, but not directly.
To explain: There are two ways of grasping a concept: by understanding what it is and by understanding what it is not. If a concept is within our sphere of experience, we can understand what it is. If a concept is outside our sphere of experience, we cannot understand what it is, but we can understand what it is not. We mentally remove it from possibility after possibility until, by process of elimination, we gain a glimpse of it.
Thus, G‑d’s statement, “My face may not be seen,” means that we cannot understand Divine providence directly, but we can understand it by negating what we know it not to be.1
Moshe is celebrated for his midot of humility. The reference to G-d's back may be a remez regarding how to individually and communally stand tall through life while enabling G-d's will. Anthropomorphically speaking, the word "back" can be figuratively understood two ways: supporting a person by having the person's back; and being angry regarding a person's conduct so you turn your back. As an aside, per Midrash, Adam and Chava were made back to back and separated to work together, ideally humbly for the good of all in life. Human existence always includes making mistakes because we are all imperfect in varying degrees. The key to best navigating life's path is learning from communal and personal mistakes to better all before finally "facing" G-d "after" life enabled by His mercy and benevolence. Hopefully, we all leave life via a kiss (Nishka) allowing us to finally face G-d ideally still standing, "B"-"H". Notably, a Tzzedek cannot stand where a B'al Chuvah stands! Shabbat Shalom!