At least currently, the presumption is that terms of a contract are understood by both parties, and there is no limit on the length or complexity of a contract. There is a risk that the writer of the contract could express a term ambiguously, in which case the ambiguity would be construed against their interest. Assuming that is not the case, then the law holds that if you agree to something, you will be held to it. Your point 3 pretty much summarizes much of the workings of the law: you are held to obey the criminal codes of the US and your state, no matter how obscure the wording is. If one is not confident of their interpretation of contractual terms, one can hire an attorney to give legal advice.
Suppose, though, that there is solid documentary evidence that the company's contract is significantly less comprehensible that contracts tend to be, and furthermore that there was a deliberate effort to make the contract especially incomprehensible – and that this effort is documented (not just surmise from the fact that they wrote an obscure contract). Suppose furthermore that one of the terms is significantly against the interests of the customer, but a normal reading of the term (by a non-specialist) would make the term seem favorable. Then one might argue that the contract misrepresented the terms, which could be a grounds for invalidating the contract. Another possibility is that the contract could be held to be unconsionable, where he leading case is Williams vs. Walker-Thomas Furniture. As that ruling says, unconscionability involves "an absence of meaningful choice on the part of one of the parties together with contract terms which are unreasonably favorable to the other party". Moreover,
The manner in which the contract was entered is also relevant to this
consideration. Did each party to the contract, considering his obvious
education or lack of it, have a reasonable opportunity to understand
the terms of the contract, or were the important terms hidden in a
maze of fine print and minimized by deceptive sales practices?
Ordinarily, one who signs an agreement without full knowledge of its
terms might be held to assume the risk that he has entered a one-sided
bargain. But when a party of little bargaining power, and hence little
real choice, signs a commercially unreasonable contract with little or
no knowledge of its terms, it is hardly likely that his consent, or
even an objective manifestation of his consent, was ever given to all
the terms. In such a case the usual rule that the terms of the
agreement are not to be questioned should be abandoned and the court
should consider whether the terms of the contract are so unfair that
enforcement should be withheld.
However, rather than articulate a principle pertaining to contract comprehensibility, the court held
The terms are to be considered 'in the light of the general commercial
background and the commercial needs of the particular trade or case.'
Corbin suggests the test as being whether the terms are 'so extreme as
to appear unconscionable according to the mores and business practices
of the time and place.'
Which hardly defines how simple a contract must be.
It is, however, necessary that the customer actually be apprised that something is an "agree" button, and the terms that they are agreeing to do have to be visible to the customer. So for example white-on-white print saying "by mousing over this spot, you agree to these terms". The button has to say more that "click to continue" – it needs to say e.g. "agree to TOS" and has to present the TOS or a link to the TOS.
