No Errata
Q: Does Sneaky Stab-stab allow me to push the upgraded fighter before each separate Attack action in an Attack action that targets multiple fighters (e.g. Whirling Halberd)?
A: Yes. However, the only fighters you can target are those who were adjacent at the start of the action (i.e. after the initial push, if you choose to use it), and your fighter must be in range of each of those fighters to make an Attack action against them. If the fighter is pushed in such a way that they are not able to make an Attack action against any of their remaining targets, no further Attack actions are resolved.