The later hail occurs in the season, the more damage it can do to yield. Hail during flowering will usually set the crop back, but additional flowering may compensate for the damage if the crop has enough time left in the season. With severe hail after flowering, plants can start to regrow, going through flowering stages again. This really sets back maturity, but with enough time, new branches can produce enough yield to make harvesting worthwhile. The question is, is this a crop worth much more investment?
Applying a fungicide after the rosette stage is too late to prevent the most serious blackleg infection and after 50% bloom is too late for sclerotinia stem rot prevention. Even if hailed canola comes back into flower, consider the potential of the crop before investing more in it.