For some reason, neither helm, ivy or selectrum allows to open a candidate buffer / file in either the current buffer, a vertical or horizontal split window, like what I have grown an addiction for with fzf.vim.
As I have also grown an addiction to org mode, emacs ergonomics always feel weird to me, even with evil mode.
Selectrum seems to explicitly exclude "alternate actions" :(
I use ivy and counsel, and both counsel-find-file and ivy-switch-buffer offer the alternate action "other window" under M-o j. I'm not sure if that covers your entire use-case, but it works fine for me.
I have come to split first and search second.
It’s all about personal preferences.
I see where you are heading and I am not following you in this rabbit hole ;)
I came to use emacs for org mode, when what I really wanted was to use org mode in vim. We can’t have All the nice things, so it’s okay, I’ll live with that. I actually found out I could use emacs as a "org mode beautifier" in neoformat.vim; it is slow, but it can tangle the org file and output back to vim. Good enough.