Is it permissible to take out an interest-bearing loan with the intention of only repaying the principal amount, and not paying the usurious increment, when one is certain of being able to avoid paying it, or when one is not certain but there is an urgent necessity?

It is not permissible to take out a usurious loan, even if the intention is only to repay the principal. Consent to a usurious contract is forbidden, and this prohibition is even stronger for someone who merely thinks they will only repay the principal. A usurious loan is only permissible in cases of necessity, and necessity has its own rulings and regulations.