Okay, so the three versions of Shada are as follows:
the unfinished serial, some of the footage of which was then used in The Five Doctors.
the Big Finish webcast, in which the events of the original Shada were undone due to The Five Doctors and so need to be relived with the Eighth Doctor.
the novelisation of the serial that, while featuring Four, seems to imply that the girl Clare is an echo of Clara Oswald. Now, for this to happen, Four’s Shada would need to be the current one, but it isn’t.
The original Shada happened, then unhappened due to The Five Doctors, causing the Big Finish version to happen in its place; that much is known. However, School Reunion implied that, for Sarah Jane, The Five Doctors never happened. There are at least two alternate solutions to this
which, for the purpose of this piece, I will be ignoring:
1. Sarah Jane didn’t count that encounter because she didn’t get a chance to confront the Doctor about kicking her out.
2. The process of being taken out of time, experiencing those events, then being placed back in time disrupted those recent memories so that Sarah was unsure if they really occurred or not.
Instead, I am going to hypothesize that the Last Great Time War and Gallifrey’s uncertain fate undid the events of The Five Doctors(I already hate myself for suggesting such a tiresome and idiotic retcon). Thus The Five Doctors wouldn’t have disrupted the original version of Shada, so the Big Finish version wouldn’t need to happen, so an echo of Clara could be, for however brief a time, slotted in via the Shada novelisation.
Scratch that, I have a better solution. With an unknown number of Claras running about between Name and its undoing in Time, perhaps another Clara intercepted the Time Scoop sent for Four and Romana. The Five Doctors still happens, but without disrupting Shada. Clara’s splinters need to compensate for the disappearance of that echo by transposing themself onto Clare and ta-dah, Gareth Roberts’ Shada novelisation gets to happen.