I know Etisalat aren't the ones under investigation, but presumably their marketing department and upper management would have some sort of document trail showing what they expected to get out of a sponsorship, and reviews of quarterly sales before and after. If it was such an excellent deal there would have been correspondence from the time between Etisalat employees.
We basically relied on CAS taking him at his word, and I know he was under oath but different people might find him less credible or think Etisalat's ownership compromises his reliability.
As others pointed out earlier in the thread, Etisalat are often seen as a small unknown company in the West, that nobody has heard of, who had no place sponsoring football clubs because they couldn't possibly afford it. In other places in the world, they are a major mobile network provider though. They don't just have annual revenues in the 10s of billions, their annual profits amount to billions too. They are probably the one UAE sponsor in all of it, who could easily have afforded to meet all their obligations, without help from their owners. Etihad are the sponsor that needed more support from their owners(as many airlines do, and it would have been expected in 2003/04 when they were created) and most of that has been cleared at CAS.
These two graphs from statista are in AED but it still amounts to billions when converted to GBP. They show the profit and revenue streams of Etisalat and although neither of them start in 2012, they do both show massive growth specifically in the 2010s, over a number of years:
Revenue streams starting from 2014 to 2022:
https://www.statista.com/statistics/1113821/etisalat-revenue/
Net profit starting in 2015 to 2022:
https://www.statista.com/statistics/1114239/etisalat-uae-net-profit/
Brand value would be another arguing point, I imagine. There is data that shows growth on that too.
Etisalat brand value starting in 2015 to 2023:
https://www.statista.com/statistics/1129926/etisalat-brand-value/
Leading telecommunications brands in the Middle East in 2023, by brand value:
https://www.statista.com/statistics/971969/middle-east-leading-telecom-brands-by-value/
Currently, they hold a fairly close second place position to Saudi Telecom, who sponsored United for many years.
Edit: Slightly off topic but I've seen some data on Etihad too, which shows there was significant growth in revenue and profits from 2011 to 2015. It was 2016, where they then started taking losses(Pep era, where the performance and brand value of City skyrocketed):
https://www.statista.com/statistics/529980/etihad-airways-revenue/
Etihad Airways key business trends(Revenues & Profit etc) 2011-2021