Doubtful that there was a second brood. No time for that at this latitude. The general timeline for grouse in central/north BC is : drumming starts in April, mating and egg-laying in May, hatching in June and growing in July/August. I've been seeing 3/4 grown chicks in the last few weeks.
If nests are lost to predators, then hens may re-nest if they weren't too far into the nearly month-long incubation period. That explains why some small chicks are seen late in the summer. Re-nesting doesn't usually happen if the chicks are lost... there's just not enough time... plus, it would be unusual if all chicks were killed at once, and the hens will stay with their remaining offspring, even if it's only one.
It's been really wet in the northeast part of the Province... doesn't bode well for grouse numbers this fall... but will wait & see