A great chance Steve ! :)
I used to play in a 7 piece function band - and as part of that had to ( briefly) understudy to an 'official' session player, before going to Germany for a week in the 'Orchestra Pit' of a local theatre group, to do 'Joseph's Amazing Technicolour Dreamcoat'. The biggest things I learned, were that the 'professionals' actually play far less than we guitarists would usually be tempted to do. More a job of 'keeping out of the way' of the Bass Player and Keyboards - whilst injecting tasty fills based on the underlying harmony.
At audition, however - I appreciate that the opposite may be true- and they may want you to play your Ar*e off / spin all the plates at once - just to see your breadth of skills and ( perhaps more importantly ) your ability to show 'grace under fire' . I assume the common thread between audition playing - and requirements for the actual show would be ( as you mentioned ) that you at least demonstrate that you understand the direction of the melody, and where you could tastefully assist the Harmony. In practice, you will of course be agreeing with the Keyboards and Bass player, how to 'Carve up' the parts of the song(s) between you - but just now (at audition ) I presume they want to see a 'Stunt guitarist' , not flash, just relaxed and reasonably confident.
The big one is of course that they want to work with you as a person - and under pressure at the rehersal , you still seem like the sort of 'Decent chap' who they could mould later. The busiest session / function / anything Bass player I used to know, was not a 'clever' player, but he always came across as a mellow guy, who took directions well - and always turned up well rehearsed. The later may seem obvious, but flash players could sometimes be inconsistent band members.
Forgive the many lines of the 'bloody obvious' - but ( subject to you checking with them ) I suspect they will be more impressed by a tastefully artful 'bit' player - than an absolutely stunning 'One man show'.
Play less, know where that sound like 'More' - and listen like crazy. I remember the 'Blagger' that I understudied to - turning up at the main dress rehersal having not even remembered the sheet music / score - for a fully orchestrated / conducted show. He just turned to me and ( tugging on his ear lobes ) said " Well, I guess we'll have to use THESE tonight " . The Conductor thought he was great - and he set an amazingly low standard - that was very hard to follow. :lol:
That may have been the (usual ) ramble, but I hope it at least pivoted on a few helpful points , that further checking with the 'Employers' and subsequent posts then start to reveal interim answers.
In any case , visibly enjoy the whole thing ; I sense you will be well recieved and are certainly 'well educated' by your Music College experience. :)
All the best ! :D