One pronoun, one noun, and one adverb

one pronoun, one noun, and one adverb

The passage below is from ‘the Blank Slate’ by Steven Pinker.

http://evolbiol.ru/blankslate/blankslate.htm

Since differences in intelligence are partly inherited, and since intelligent people tend to marry other intelligent people, when a society becomes more just it will also become more stratified along genetic lines. Smarter people will tend to float into the higher strata, and their children will tend to stay there. The basic argument should be banal because it is based on a mathematical necessity: as the proportion of variance in social status caused by nongenetic factors goes down, the proportion caused by genetic factors has to go up. It could be completely false only if there were no variation in social status based on intellectual talent (which would require that people not preferentially hire and trade with the talented) or if there were no genetic variation in intelligence which would require that people be either blank slates or clones).

I'd like to ask three questions regarding the underlined words.

First, about the meaning of 'argument'.
I looked up FreeDictionary as follows:

http://www.thefreedictionary.com/argument
1. reason, case, reasoning, ground(s), defence, excuse, logic, justification, rationale, polemic, dialectic, line of reasoning, argumentation
? There's a strong argument for lowering the price.
2. debate, questioning, claim, row, discussion, dispute, controversy, pleading, plea, contention, assertion, polemic, altercation, remonstrance, expostulation, remonstration
? The issue has caused heated political argument.
3. quarrel, fight, row, clash, dispute, controversy, disagreement, misunderstanding, feud, barney (informal), squabble, wrangle, bickering, difference of opinion, tiff, altercation
? She got into a heated argument with a stranger.

Even though 'argument' is used as meaning #2 'claim' the most, it seems to be used as #1 meaning, reason, ground, logic, and rationale, in this context.

(Am I right?)

Second, about what the underlined 'it' represents.
It seems to represent the underlined two IF-clauses.
(Am I right?)

Third and last, the meaning and role of underlined 'only'.
I'm not sure why 'only' is used in this passage.
It seems to be used the same as the example sentence as follows:
? The bank will lend you money only if you sign the paper.
(Am I right?)

But, if it's right, I think the sentence with 'only' can be used without any difference of meaning without it.
(Am I right?)

If my thought is right, why the author used 'only' in this sentence?

(Sorry to bother you for so many questions in a thread. But since they are in a same paragraph, I couldn't think of any better way of posting my thread.)

Regards.