From what I understand about US tax law, Delaware is basically a mini Luxembourg and is favoured for it's shareholder anonymity so yeah, it doesn't surprise me that the duty for disclosure is considerably less!
If I remember correctly you are a US tax lawyer so you might get a kick out of the level of statutory disclosure required for limited companies in the UK -
https://find-and-update.company-information.service.gov.uk/company/02570509 that's the Companies House page for one of the private limited holding companies in the United group. The information contained on a private company's statutory accounts and on directors and shareholders will probably surprise you!
Your question about how foreign holding companies fit into the picture is a complex one to answer. It's true that a number of clubs are owned through overseas holding companies but all of these structures will still have a holding company in the UK which needs to statutory disclosure criteria. In short it is very unusual for football clubs (or any company that generates profit in the UK) to engage in the sort of base erosion or profit shifting that Amazon / Facebook etc are famous for because of our tax law. To grossly over simplify our controlling foreign company tax laws, revenue generated in the UK generally can't be siphoned to other jurisdictions to prevent paying tax. There was a good study on the corporate structure of UK clubs by the Tax Justice Network. To paraphrase, the corporate structure of UK clubs is designed to allow foreign entities to funnel money into UK companies with minimal oversight and shareholder anonymity, but not to funnel profits out. Foreign holding companies bring the larger existential tax avoidance issue for sale or disposal, but there's currently no evidence that UK clubs are extracting operating profit to pay dividends via foreign parent companies. Again, it's pretty unusual for football clubs to declare dividends full stop.
I do agree with your conclusion that they aren't doing anything illegal or that unusual for corporate finance. It is however very unusual for a football club. The LBO and hostile takeover was unique in football as far as I'm aware. So is the level of debt in the club to non connected parties (except for spurs, but they borrowed to build a stadium). So is the dividend policy and the level of money they extract. As was the decision for a public share offering to refinance expensive PIK debt.
One question which you may be the perfect person to answer... as you're likely aware, United the PLC are incorporated in the Cayman Islands. Most reporting say that although this will increase the Glazer family's personal tax bill (for the reason I stated above - they'll be paying tax twice in some cases), it was done to give anonymity and a sweeter deal to new public shareholders. This makes sense since they moved the club to the Cayman Islands shortly before the decision to publicly offer stock. However... The Glazer family ownership of the PLC is also notoriously complicated and is constructed of several family trusts, almost all incorporated in Nevada. Does any reason jump out at you why they'd pick Nevada rather than say, Delaware?
It's a tiny point, but the club have always refused to comment on why their corporate setup is structured the way it is, and it's always annoyed me that I can't see the logic of family trusts in Nevada.