Yes, you add interest income to your stock basis.
When you refer to box 17A, I assume you mean Box 17A of Schedule K. Box 17A includes all investment income, not just interest and it is used to calculate your investment interest limits on Form 4852. It is not a contradiction, just a different use.
As for the pass through income, that is the foundation of shareholder's stock basis. The stock basis keeps track of income on which taxes have already been paid. The main use of the basis is to determine if distributions are taxable to the shareholder or are a return of principal.
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