No.
I say this with such certainty because I personally had the same situation with a client several years ago. The IRS audited and disallowed the guardianship prior to the adoption because they said it was not "for the principal purpose of the legal adoption." I felt certain, like you seem to, that the correlation between the adoption and the guardianship made sense, but the IRS ruled otherwise.
The IRS defines qualified adoption expenses as:
Qualified adoption expenses are reasonable and necessary expenses directly related to, and for the principal purpose of, the legal adoption of an eligible child.
Qualified adoption expenses include:
Adoption fees,
Attorney fees,
Court costs,
Travel expenses (including meals and lodging) while away from home, and
Re-adoption expenses relating to the adoption of a foreign child.
