According to your figure, we have the following BDD nodes (I guess the missing low-edge of N4 points to 0?):

N2 = (a => 1 | 0)
N3 = (b => 1 | N2)
N4 = (c => 1 | 0)
N5 = (c => N3 | 0)
N6 = (d => N4 | N5)
E2 = (a => 1 | 0)
E3 = (d => E2 | 0)

I think that the computation should be as follows:

Exists(E3,N6)
= Exists((d => E2 | 0), (d => N4 | N5))
= OR(Exists(E2,N4),Exists(E2,N5))
= OR((c => Exists(E2,1) | Exists(E2,0)),Exists(E2,N5))
= OR((c => 1 | 0),Exists(E2,N5))
= OR(N4,Exists(E2,N5))
= OR(N4, (c => Exists(E2,N3) | Exists(E2,0)))
= OR(N4, (c => Exists(E2,N3) | 0))
= OR(N4, (c => (b ? Exists(E2,1) : Exists(E2,N2)) | 0))
= OR(N4, (c => (b ? 1 : Exists(E2,N2)) | 0))
= OR(N4, (c => (b ? 1 : OR(Exists(1,1),Exists(1,0))) | 0))
= OR(N4, (c => (b ? 1 : OR(1,0)) | 0))
= OR(N4, (c => (b ? 1 : 1) | 0))
= OR(N4, (c => 1 | 0))
= OR(N4, N4)
= N4

Right?