Tool for obtaining a BibTeX entry from a key, without creating it manually
I doubt it. In all but the most verbose or non-human friendly (e.g., DOI) key style, the problem is a one-to-many mapping in that each BibTeX key maps to many sources even though any single source has just a single BibTex key. Even in relatively small personal bib files, conflicting keys are common and need to be disambiguated (usually with an additional letter or number).  The bigger the database (and Goggle Scholar is a big database) means that there will be lots of collisions.