Susan is an important part of the plot. The MC has 'by accident' (or due to his addiction) intruded on a special relationship between Rachel, Susan and Uncle.
So the MC has to adapt to the circumstances and Uncle has decided - obviously after a discussion with Rachel ( and probably Susan was involved) that the MC is a suitable candidate to give Rachel what he (Uncle) cannot.
So Susan knows MC has permission to fuck Rachel as long as Rachel wants it.
So Susan has to also adapt.
She has already said "I like that boy"
She has also already said: "You will have to be a very good boy to use that" (referring to his 'real' cock over a plastic one).
It is likely that at some stage the MC will get regular access to Susan's room during Rachel's punishment time as was possible before 'The Island'. So we would have the three of them interacting.
If the MC treats Rachel very well, then that will endear him even more to Susan.
I don't think it beyond the realms of reason that Susan might not be tempted to 'try' the MC's cock out herself. OK he may be 'restrained' on the bed so Susan has control, but if she likes it, then who knows what might happen and just who might end up controlling who?
There is always the possibility that Susan might break 'the rules' herself and 'earn' a spanking - she might even enjoy it if the MC proves he's man enough to control her.
It's a porn game - and so as long as the plot development is not 'sudden' and totally beyond belief, then the experienced Domme is not unassailable.
It might be the final 'Big Boss' but not beyond the possibility.