International sporting events need not be fiscal disasters, the financ

You're right -- it's definitely plural, not singular. Answer D also does not contain a comma splice (the post you quote seems to suggest it does).

Answer D is grammatically fine, but it has two issues that make E preferable. Answer D is passive for no good reason ("a fact demonstrated by the Games" is passive, while "the financial success... demonstrates" is active), and answer D is a lot wordier than necessary. If answer E weren't there, D would be the right answer, but sometimes in GMAT SC, we're choosing between two grammatically correct choices, and picking the answer that most effectively and precisely expresses the idea of the sentence. That's what we need to do here.

In addition to what Ian said above, there is one more reason I thought (E) is better -
What demonstrates that international sporting events need not be fiscal disasters? The financial success of 1992 games, not the 1992 games themselves.
But that's an ideal situation.
I concede that we could say, "The games illustrate that ..." but then we need to explain that the games were a success. But then there is a disconnect between the games' success and what the games demonstrate. We want to say that the success demonstrates ...
Hence option (D) isn't the best.

Option (E) is just so much clearer and to the point.

International sporting events need not be fiscal disasters, the financial success of the 1992 Olympic Games demonstrates that fact.

(A) the financial success of the 1992 Olympic Games demonstrates that fact (incorrect: run-on sentence)
To connect two clauses, there should be a conjunction that is missing in option A.
(B) for example, the 1992 Olympic Games were financially successful (incorrect: placement error)
The underlined part is not an example of a disasters.
(C) like the financial success of the 1992 Olympic Games demonstrates (incorrect: comparison error)
Like is used to show similarity between two nouns instead of two clauses
(D) a fact demonstrated by the 1992 Olympic Games, which were financially successful (incorrect: modifier error)
A fact will become a modifier for disasters that is incorrect.
(E) as the financial success of the 1992 Olympic Games demonstrates (the best option)
x as y: events need, success demonstrates