uncountable noun with article 'a'

English as a Second Language English Vocabulary, Grammar and Idioms
1
  1. I didn’t expect you to do such (X: a) work(UN).
  2. It is such a pity(UN) that that man has died.
    In the above sentences, ‘work’ and ‘pity’ are used.
    But both words are uncountable nouns.
    How is then in #2 the article - ‘a’ used in front of ‘pity’?
    Please explain with its relevant grammar details?
2

Because in the second example the word “pity” is used in one of its coutable meanings:

Pity = a matter of regret, an unfortunate chance.
What a pity you can’t come
thefreedictionary.com/pity

3

Beeesneees,

  1. In order to accept your theory as fact, they would like to see some form of scientific evidence.
  2. In order to accept your theory as a fact, they would like to see some form of scientific evidence.
  3. In order to accept your theory as fact, they would like to see some form of scientific evidences.
    Are all these correct?
    If some are wrong, please explain.
4

3 is incorrect. ‘Evidence’ is uncountable.