Izzy does indeed fly prototypes on development work, although not as captain on envelope expansion tests. The ETPS course she completed was the then Diploma Course, which was progressively developed over the years until being dropped as the EASA FTL appeared. The syllabus included performance and flying qualities, including variable stability training, and was therefore wider than the more recent EASA production tp syllabus. Several past graduates of the Diploma Course were immediately used as experimental test pilots and FTEs by their companies after graduation.
I'm not sure what Reinhardt is getting at with his comment about paying for training. All courses at ETPS (and the other schools) are paid for by someone, be they private individuals, companies, or other organisations. If he's implying that Izzy bought her graduation, then the same must apply to all pilots who have paid for training of any sort, be it for a PPL, and ATPL, or a type rating. That doesn't mean that the qualification is guaranteed. ETPS has a proud reputation to maintain and graduating sub-standard students would be a sure way to lose it.
The test flying training world has changed a lot in recent years, and there are (and actually always have been) many different ways to achieve the end goal of a career as an experimental test pilot or FTE. Some (like me) are fortunate enough to have training offered to them for the taking via the military, others have to build up to the qualification via other routes. Either way, a huge investment of personal effort is required.
In fact, even major companies such as Airbus do internal development of pilots and FTEs, as well as hiring pre-qualified people. The former gives long-term in-depth knowledge of the company's products, whilst the latter brings fresh ideas. Both aspects are valuable.
If you want to make a career in flight test, you can. But you may need to be brave in your choices... Be bold!