KCL equation considering the leakage current
Since you are writing a sum of currents this is quite straightforward:

Vb: sum of all currents leaving that node

which can be written as

Vb: (Vb-Va)/R2 + Vb / R5 + Ileak + (Vb-V2)/R3 = 0

this can be rewritten as

Vb: (Vb-Va)/R2 + Vb / R5 + (Vb-V2)/R3 = -Ileak