Hi,
The current voltage I-V sweep for secondary electrons SEE takes into account SEE from all spacecraft surfaces. At Vb = +20 V, all SEE emitted by the probe may be recollected you're right. But I don't agree with your analysis of SEE from SC being repelled directly to infinity. 2 eV SEE from SC can be collected by the +20 V biased probe, even if the spacecraft is negative. A large spacecaft emits a large current and even a small fraction of it recollected by the probe may by significant for the probe, whose measurement is totally polluted.
So to me, this is not a probem of SEEE yield computation.
Hope this will help you.