Learn Italian: Diverse impalcature non stanno venendo smantellate. - Several scaffoldings are not being dismantled.

You should avoid it. While stanno essendo smantellate is technically understood, a gerund followed by another gerund or participle often sounds extremely heavy and unnatural to Italian ears. For the progressive passive (meaning "is being done"), Italians almost exclusively use stare + venendo + the past participle.

Does diverse always mean "several"?

Not always! Its meaning changes depending on its position. When placed before the noun, as in diverse impalcature, it means "several" or "various". If placed after the noun, like impalcature diverse, it changes to mean "different" or "distinct".

Why does the past participle smantellate end in -e?

In any passive construction, the past participle must agree in gender and number with the subject receiving the action. Because impalcature is a feminine plural noun, the participle must take the feminine plural ending and become smantellate.

Where does the negative word non go when the verb phrase is this long?

It always goes directly in front of the conjugated verb. Even though the full verb phrase here is three words long (stanno venendo smantellate), the non simply sits right before the conjugated helper verb, stanno.