Is it permissible, in Islamic law, to pay a small portion of the taxes due on a company in a country rife with corruption and injustice, given that the vast majority of companies do so?

People's consensus on a matter does not make it permissible if it is forbidden. Therefore, the conspiracy of traders to evade taxes does not make it permissible. One must look at the taxes themselves: if they are permissible, then it is not allowed to evade them. However, if they are oppressive and unjust, then there is no harm in evading them to the best of one's ability, as long as one does not expose oneself to humiliation and harm, for the Prophet, peace and blessings be upon him, said: "It is not proper for a believer to humiliate himself." They asked: "How does he humiliate himself?" He said: "By exposing himself to trials that he cannot bear."