Passed right through the depths of my heart: Ok so the term he used here can refer to the lungs, but I doubt he's talking without lungs. The term can be also interpreted as 'from the depths of my heart' in an emotional/heartfelt sense. I'm pretty sure he's actually making a joke about how Yuchi's stab hurt his heart both emotionally and physically.
I think this is the first time its being mentioned that there are no seniors 1-3 anymore which makes sense since they've never appeared. Unfortunately it seems that number 4 is about to go as well
Since Long Nu died, I always had the feeling Muxian (the 4th bro) was gonna die pretty soon as well. Because it felt like he deeply regretted her death. They had a promise that he never had the chance to fulfill because of her death. Maybe deep down he wanted to die to follow after her.
edit: i noticed that in earlier chapters, the demonic entity that fought with Spellbane's owner was a Great Ape Sage. so is the evil cultivator from an ape clan related to that sage?