The worst thing about Mary's financial arrangements is that it's all so phony. Peter and Gerald have control of her money, but if Mary came to them and said "Look, something's happened, I need my money," do you honestly think either of them would say no? So the only thing keeping her from the money is her own scruples, and if her scruples are enough why bother with the legal rigamarole?

I like the idea of Harriet's money keeping them in the style to which Peter is accustomed after the war.