Only volume 17 has a shortage problem, given that we still don't have a reprint announcement for it, but all the other volumes are getting urgent reprints that are going to bookstores next week and those are still selling like hotcakes. OP's volumes do sell way more than any other series, but KNY's backlog is super strong and since they are sending out reprints, it doesn't seem it will stop soon.
Dude, you're missing the point completely. The times OP recently trended was from leaks, which means they are released online way before the maganize's physical release. Japanese people despise those leaks, they constantly block people that post those on Twitter and remind people not to post them and Jump/Shueisha has tried time and time again to stop them from being uploaded. Also, it trended outside of Japan and we're talking about Japanese sales. There's no direct correlation between those trends and sales. Oh yeah also, the chapters that trended? They are only going to be compiled in volume 95.
Average. But it's kind of a delicate situation, honestly. JJK which only has 7 volumes out is super close to BC when it comes to weekly sales, while BC has an anime with over 100 episodes and has 22 volumes out and doesn't have a significant backlog. SXF, which has only 2 volumes out, already sells more than BC. It has almost no room for a boost since the anime, which has had better production for a while now, does nothing for the series's sales and there's no sign of a movie, which could help. BC's sales outside of WSJ are not bad per se, but within WSJ, they are mid tier.