Is a partner's purchase of his brother's share of a building with bank financing, followed by the brother's purchase of two adjacent plots of land for himself and his partner with the money paid by the bank, considered a circumvention of usury or does it involve a legal doubt?

There is no objection to what you intend to do, provided that the banking transaction adheres to Sharia guidelines, and that you do not deceive the bank by telling them that the building belongs to your brother so that he may buy it and then sell it to you, which would lead to a forbidden 'inah sale. The correct approach is for the bank to buy only your brother's half. Once they own it, they can sell it to you, and the entire building will then become your property. There would then be no objection to your agreement with your brother to buy land and build on it.