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.
Q: Can I use Sneaky Stab-stab to make an Attack action when my fighter is not in range?
A: Yes, but only if the one hex push takes your fighter into a hex where they have line of sight to and are in range of the target. You cannot use Sneaky Stab-stab if you can’t push the fighter within range to make their Attack action with the one hex push granted by Sneaky Stab-stab. If something happens (e.g. a reaction to the push) so that the fighter is no longer in range to make the Attack action, the Attack action is not made (it does not fail or succeed) and the activation ends. No reactions can be made that are triggered by an Attack action and it does not count towards objectives that count Attack actions made (because the Attack action is not made).