If you really want to comply with the letter of the law (and its unintended consequences in cases like this), the procedure is right there in the Form 8615 instructions.  Don't say the student will never have access to the parents' tax information, because the law allows disclosure by IRS.  

The tax information of the guardians has nothing to do with their ward's return.  It's unfortunate that they didn't adopt, but there were probably good reasons that was not practical.  (Are they in a state that allows adult adoption?  IRS might accept that.)

This close to the extended deadline, I would use a high estimate and then file a claim for refund using zero taxable income for the parent(s).  Allow IRS a year or so to sort it out, and get an early start for the 2025 return.  

If following the instructions doesn't yield satisfactory results, it's a good case to turn into a Congressional inquiry.  Ask the people who voted for it.  

Parent's return information unavailable. If a child can’t get the required information about their parent's tax return, the child (or the child's legal representative) can request the necessary information from the IRS.

How to request. After the end of the tax year, send a signed, written request for the information to the Internal Revenue Service Center where the parent's return will be filed. (The IRS can’t process a request received before the end of the tax year.)

The request must contain all of the following.
• A statement that you are making the request to comply with section 1(g) of the Internal Revenue Code and that you have tried to get the information from the parent.
• Proof of the child's age (for example, a copy of the child's birth certificate).
• Evidence the child has more than $2,600 of unearned income (for example, a copy of the child's prior year tax return or copies of Forms 1099 for the current year).
• The name, address, social security number (SSN) (if known), and filing status (if known) of the parent whose information is to be shown on Form 8615.