Abbreviating first names in the bibliography
At the end of the day this all boils down to opinion and you already gave the most important argument yourself: Usually, the only way to obtain consistency is to abbreviate all first names. If you try to use unabbreviated names even when the journal doesn’t, you will eventually run into a case where you cannot find out an author’s full first name by any reasonable means.

All journals that I am aware of adhere to such a citation style, so you would be in good company abbreviating first names. Moreover, such a citation style would not arise if it would lead to any practical problems. In fact, knowing an author’s full first name rarely accelerates finding a cited publication, and if then only by a little. I cannot imagine a situation where the first name would actually be necessary to find or identify a properly cited paper – you always have page numbers or similar to resolve potential ambiguities.