Not sure if this is relevant or not, but I'm pretty sure the equivalents designs for cannons (e.g. on the walls of Fort George in Scotland) are as described and are widest on the outward part.
Sure, but those usually have a gun shield protecting the gun crew & it is done due to the gun barrel being long & the gun being heavy, so it is better to mount it on a single point, instead to jave it traverse on a curve (which is not a problem for an archer).
Not that the apertures are stepped instead of smooth - I guess the avoid the funneling effect mentioned above, possibly changing a near miss to deflection and hit on the gun shield. Should not happen if the steps get in the way.
Apparently not [0]. IANA cannon expert but I think in the 18th C their biggest threat would have been from cavalry rather than sharpshooting infantry or precision fire from other cannon.
https://en.wikipedia.org/wiki/Fort_George,_Highland