What is the ruling on dealing with and working in usurious banks in countries where there are no Islamic banks available, and all institutions therein rely on usurious transactions, despite the existence of limited alternatives such as the postal bank or the black market, and what are the consequences of this for individuals and societies?

Working in usurious banks is forbidden, whether or not there are Islamic banks in a country. It is not permissible to deal with them except in cases of extreme necessity. Necessity does not include what you mentioned about the factory owner and the black market; these are invalid justifications. A Muslim must avoid usury, as the consumer of usury, its giver, its scribe, and its two witnesses are cursed, as stated in a Hadith (definition) in Muslim. As for unlawful taxes, it is permissible to evade them, provided that this does not lead to a greater harm.