Best Windows tool for managing .bib files to be later processed by biblatex?

Jabref can do it very easily. You can change Jabref settings to handle biblatex entries in this manner:

In the Jabref window, press the menu Options and then Preferences

The preferences window will appear:

In the submenu Advanced, find the BibLaTeX mode (the last item) and check it. Then press OK. Now Jabref will handle .bib entries suitable for use with BibLaTeX.