You will need to supplement Duo with another resource to have the concepts explained rather than demonstrated. The only reference to the 6 cases I've seen here is in the section titles. There are many good references I've found on the internet, google "Russian cases" and go from there.
Yes. Also, neuter prepositional, which is more appropriate here ("это" just refers to "some stuff" and isn't a modifier before a noun).
Masculine and neuter adjectival declensions are the same everywhere except the Nominative (and Accusative when Accusative copies the Nominative). This also work for most cases of neuter and zero-ending masculine nouns—but no so perfectly because the ending of the dictionary form will matter in the Nominative and Genitive plural..