1. For the transesterification reaction, your ester is the reactant present in excess, so all of the 3,5-dinitrobenzoic acid should react away. If, however, the 3,5-dinitrobenzoic acid was used in excess, how could the excess be removed after the reaction is complete

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.