Thinking about the problem I want to use this for... I think I need ⊐⟜(-1) something to start with.
With that, and some searching on BQNCrate, I've made some improvements to my solution, and it's closer to supporting part two. At least, I think they're improvements? I really need a better sense of idiomatic style for APL-like languages.
Anyway, I spent enough time trying to figure out array manipulation, better wrap up for now.