Short answers, pulled from the story.
A mass noun is a noun whose quantity is treated as an undifferentiated unit rather than as something with discrete elements. In English, mass nouns cannot be directly preceded by a numeral without a unit of measurement ("twenty litres of water", not "twenty water") and cannot be combined with an indefinite article such as "a" or "an".
Two tests identify mass nouns in English: the word cannot be modified directly by a numeral without a unit of measure, and it cannot take an indefinite article. Mass nouns also take quantifiers like "much" and "little", while count nouns take "many" and "few". Both types accept "some", "all", and "no".
Cumulativity, formalised by Manfred Krifka in 1998, means that combining two instances described by a term still produces something described by that term. Water added to water is still water; cutlery added to cutlery is still cutlery. Linguists who prefer this definition characterise mass nouns as cumulative nouns and count nouns as non-cumulative nouns.
Yes. Words like "paper", "coffee", "chicken", and "fire" regularly function as both mass and count nouns depending on context. "Paper" is a mass noun as a material but a count noun for academic submissions. "Coffee" can appear as a plain mass noun ("I'll have coffee") or a count noun ("two coffees").
Finnish marks the distinction through grammatical case: the partitive case signals an indefinite quantity ("join vettä", I drank some water) while the accusative case signals a specific bounded portion ("join veden", I drank the water). In Chinese and Japanese, some linguists argue all nouns behave as mass nouns, requiring a measure word before any numeral.
No. Collective nouns such as "group", "family", and "committee" are a special subset of count nouns, not mass nouns. Both types can show singular verb agreement, but for different reasons: mass nouns use singular forms because their substance is grammatically indivisible, while collective nouns shift between referring to the group as a whole and its individual members.