> 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.

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.