More than two is a bit painful, in my experience, as I don't think Emacs has directional focus like the usual X11 WMs (e.g. i3, bspwm) and C-o works well with only two.
Magit and sly/SLIME work much better with a second window, I'd say.
It does, but I don't think they have default bindings. The commands are windmove-{left,down,up,right}. I've had them bound to C-S-{h,j,k,l} for years and it makes moving around much more fluid and predictable than C-x o.
Magit and sly/SLIME work much better with a second window, I'd say.