Learn Farsi: هیچ‌کس آنجا نبود و اوضاع کاملاً عادی است. - Nobody was there and the situation is completely normal.

In Farsi, negative indefinite pronouns like هیچ‌کس (nobody) or هیچ‌چیز (nothing) always require a negative verb, even though the English translation uses a positive verb. This grammar rule is called negative concord or a 'double negative'. So, this sentence literally translates to 'Nobody was not there.'

اوضاع (owzâ') is a plural word, so why does it use the singular verb است (ast)?

اوضاع is the Arabic broken plural of وضع (vaz' - situation), meaning 'situations' or 'conditions'. However, in Farsi, inanimate plural subjects usually take a singular verb. Since a situation is an abstract, non-living thing, the singular verb است (is) is used instead of the plural هستند (are).

What is the double line on top of the alef at the end of کاملاً (kâmelan)?

That symbol is called 'tanvin', and it is borrowed from Arabic. It is added to the end of some words (written over an alef) to turn them into adverbs, and it is pronounced as '-an'. Here, it turns the adjective کامل (kâmel - complete) into the adverb کاملاً (kâmelan - completely).

Would Iranians say هیچ‌کس (hich-kas) exactly like this in everyday conversation?

In casual spoken Farsi, هیچ‌کس is very often shortened to هیچ‌کی (hich-ki). You will usually hear 'هیچ‌کی آنجا نبود' (hich-ki ânjâ nabud) in everyday, informal situations, while is more common in writing or formal speech.