Grammatically speaking, it should be 'them', but notionally speaking, many native speakers would subconsciously conceive of the group of spectacular things as a single unit against which to compare 'this'. They would then use 'it'.

He tried to divide others opinions, attitudes, etc, instead of uniting them.

What could be wrong with it? The phrase as it stands in isolation is OK, but it may not fit into whatever sentence you plan for it.

'Most least' juxtaposed is extremely poor style. Use the first one.

Dear Sir or Madam, We would like to get your comfirmation that your case number 13 has been resolved. Kindly reply to this email so that we can close this case. Thanks and regards , IT support team Dear Sir or Madam, This is to inform you that

Derelict = abandoned Decrepit = in rundown condition

Use which or that for restrictive clauses. (Some grammarians prefer that only.) Use which only for non-restrictive clauses. For more information, google those phrases.

1-- No; change in policy, I think. 2-- Yes.

The message seems straightforward to me; it contains no idiom. Which words confuse you?

It sounds as if his children do not want to be near him, so the co-worker wonders what is wrong with him.