Unless there was some procedural error that makes it necessary to do justice, it will not be a rehearing. It will be a review of the original decision. The focus will be on the decision that was made and the reasons given for it rather than this bit of evidence, that bit of evidence, and t'other bit of evidence.
Given the misconduct appears to have been admitted, it is unlikely that there was much in the way of evidence anyway. The basic purpose of evidence is to resolve factual disputes. Where there are admitted or agreed facts, the decision maker applies the rules to those facts.
If the only issue is the proportionality of the sanction, then they should be able to hear all the arguments and render a decision pretty sharpish. I've done much more complicated appeals in less than a day, with a decision given then and there.
Obviously it is not my decision to make, so my views are irrelevant. But I don't envy Southampton's legal team. It is likely that they will have to convince 2 out of 3 of the arbitrators (who will all be barristers, solicitors, or professional arbitrators) that the decision was one that no reasonable decision maker could have made. It won't (shouldn't) be enough to convince the arbitrators that they themselves might have made a different decision in the shoes of the original decision maker. A tricky task.