A forced fermentation test would indeed be useful. It will tell you if the yeast crapped out prematurely or if they simply finished attenuating the wort. If the former, adding more yeast (krausening) is the best choice. If the latter, you could add ultra-ferm. This would resume the attenuation without imparting flavors from some other yeast strain. Once the gravity and flavor are where you want them, cold crash asap to halt further yeast activity.
As a side note, that beer is 8.4% abv. Seems risky (?) to culture from a bottle that is who knows how old and this high an abv. I wouldn't be surprised if the yeast simply couldn't get over the finish line. And is the bottle dregs the same strain that was used to ferment? Not uncommon to use a different strain for bottling, although admittedly I don't know anything about this beer.