About showing/telling, it's not just nen exposition where it happens. But for example, Hisoka was using cards to slice since his first appearance. We don't need a whole explanation and name for the technique ("shu" IIRC.) Other series do the same thing (imbuing ki into objects) without saying anything.
That particular training was pointless mostly because it didn't really introduce anything new. The fights/results weren't necessarily pointless.
You could make the argument that those characters ended up accomplishing nothing either, but that's not really important.
HxH just reads like a diary more than anything. It's about things that happen to the characters. If the story points out that Kurapika's revenge is hollow, or that Gon is selfish, it doesn't change that fact. There's rarely another side to the story.
What makes something "not shallow"? I don't really feel like HxH is attempting to defend itself on that front. Characters like Wing and Biscuit exist because the plot required them, and that's fine with Togashi. Those characters don't have a stake in anything else going on, and the actual development only happens to a very small group
I dunno, I don't think it's worth the time digging into details for this. The best I can give is impressions. Though, I do feel like One Piece makes an effort to have its characters feel like they would still exist without the plot.