Learn Farsi: یک توده سهمگین از ابرها و یک جبهه تاریک آسمان را پوشاند. - A formidable mass of clouds and a dark front covered the sky.

In Farsi, subjects that are inanimate objects usually take a singular verb. Even though there are two inanimate subjects here joined by va (and), they are treated as a collective non-human force, so the third-person singular pushând is used instead of the plural pushândand.

Could we just use an Ezafe instead of az and say tude-ye sahmgin-e abr-hâ?

Yes, an Ezafe chain works perfectly there. Using az (from/of) instead is a stylistic choice that emphasizes what the mass is composed of (a mass made out of clouds). This structure is very common in Farsi for quantities, containers, or collections, similar to a phrase like yek quti az shirini (a box of sweets).

I recognize the -gin ending in sahmgin (formidable). Is this the same suffix used in basic emotions?

Exactly! The suffix -gin attaches to nouns to form adjectives meaning 'full of' or 'characterized by' that noun. Here, sahm (fear/awe) becomes sahmgin (fearsome/formidable). You have seen this since your beginner classes in words like ghamgin (sad, full of sorrow) and khashmgin (angry, full of anger).

What does jebhe mean outside of a weather context?

Jebhe literally means a 'front'. While used here for a weather front, you will very frequently hear it in political or military contexts to mean a battlefront, a war zone, or a political coalition.