As far as I understand it the reason for having so many companies is to ensure that liabilities and assets can be sold on/disposed off or purchased with affecting other property on the site or under the auspices of DLP.
For example, the convention centre could go bankrupt and be forced out of business WITHOUT the liabilities of that segment of the resort affecting the rest of the place.
Equally, should the debt mountain grow too large Euro Disney SCA would be a sacrificial lamb.
It would enter administration and TWDC take over the running of the park, effectively eliminating the debt.
This would only happen in extreme circumstances.
It is common practice for large organisations in debt to run umpteen companies all of which could go bankrupt before the actual owners either lose money or control.
Another example is Formula 1. A business that is billions of dollars in debt, yet has been sold by the same individual 3 times, all the while that individual (Bernie Ecclestone) remains in control!
F1 is nominally owned by the CVC group, yet the debt is split between the Topcon (Alpha thru Delta) companies.
It would appear that Disney operates in the same way.