When I did CA2 we were provided with an example mark scheme on the first day. Although this will naturallly vary from one project to another, you can see the mark allocations in each section, e.g. audit trail, spreadsheet model, etc. so you should be able to get a feel for the answer to your question. In the exam, time pressure will probably dictate that you have one go at getting the model right, without too much debugging time, and then move onto the audit trail and documentation.
One final thought: on the first day when I took CA2, the supervisor told a story of someone in a situation of the kind you describe, i.e. excellent written work but fundamental mistake in the model. The supervisor said he just about managed to award a pass, BUT if only the candidate had asked the supervisor during the exam about the issue he slipped up on the whole problem could have been avoided.