Subject Verb Agreement : Use of AND

getgyan and jennyv,
I must apologize. I am revising my position on sentence #3. I did quite a bit of checking, through contradictory sources, but the source I am taking as authoritative is the Oxford English Grammar, by Greenbaum, a truly extraordinary reference work.

First of all, this grammatical construction bears a few different names --- "noun clause", "substantive clause", and the Oxford book refers to it as a "nominal clause." These are full (noun + verb) clauses that, as a unit, act as noun and occupy one of the noun-roles in a sentence. When a substantive clause, or nominal clause, is the subject, then in the overwhelming majority of cases, it is construed as singular and takes a singular verb. That much is probably all you need to know for the GMAT SC. Probably everything beyond this is beyond the GMAT.

There is an exception to this rule --- what the Oxford book calls a "relative nominal clause" --- that is a nominal clause that begins with a "nominal relative pronoun". The ten nominal relative pronouns are:
who, whom, whoever, whomever, whosoever, which, whichever, what, whatever, whatsoever
(I would bet every donkey on planet Earth that you will never see the words "whosoever" or "whatsoever" as a crucial deciding word in a GMAT SC problem!)
Unlike other substantive or nominal clauses, a relative nominal clause don't automatically take the singular --- they can take the plural if the nominal relative pronoun is interpreted as plural. That's the crucial point: how is the number (i.e. singular or plural) of that pronoun understood?

Let's look at this sentence

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating, but what ____ much more difficult to determine _____ the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.

We need to fill "is" or "are" into each of those two blanks. First of all, whatever goes in one blank must go in the other as well --- if we determine the pronoun "what" is singular, that requires a singular verb inside the clause, and it makes the entire clause function as a singular noun, so the other blank will have to have a singular verb as well. Likewise, when both are plural. Ultimately, the word "what" refers to that list of three things, so it must be plural, and therefore both verbs must be plural.

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating, but what are much more difficult to determine are the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.

Sometimes, when the nominal relative pronoun does not act as the subject inside the clause, we need to look what follows the verb to determine the number of the pronoun. For example,

What I find objectionable is all the noise during the film.

What I find objectionable are all the people who talk during the film.

Where he lives is a small hamlet outside Stockholm.

Where he lives are fourteen different caves on the cliffs off the Dingle Peninsula.

Whomever you heard making noise in the courtyard at 3 a.m. was most likely a deranged individual.

Whomever you heard making noise in the courtyard at 3 a.m. were arrested and taken away in several police cars.

Notice, with all due respect to the "slideshare" website --- it's a little more than just noun clauses that begin with "what" --- technically, such a clause could begin with any of the nominal relative pronouns listed above.
Ultimately, in some cases, the only way you would know the correct number of the verb is by understanding the context. After all, int he big picture, GMAT SC is an artificial task: correcting a sentence out-of-context. What matters in the long run is how we use grammar in the world: when you know the context, you will know the proper number, and you can determine the number of each verb accordingly.
NOTICE: we are in the stratosphere of English grammar here. Deciding the correct verb number following a nominal relative clauses--- I can absolutely guarantee this will not be a task on GMAT SC. Even the idea of a simple noun-clause as the subject of a sentence is relatively unlikely to appear.

Does all this make sense?

Mike