It may help if you read the RNS's about the security of costs. It is EPI who have paid into the court NOT SYM.
The 75% figure you quote is also misleading. What I think you are referring to here is the normal procedure for the legal teams ( both ) to submit to the court their costs which are then scrutinised by both sides and usually reduced by some 30 % having referred to the table of costs. So the fees are reduced by agreement/negotiation by some 30 % and NOT, as you are trying to suggest, 30% of the costs left unpaid.
Where I do agree with you is that the case could go either way and is dependent on the strength of the legal teams. However, having already won the main case convincingly, it will take some spectacular new evidence to sway the three Judges and persuade them that this appeal is nothing more than an EPI scam.