Flat head rivets, I must admit that in all my years in the industry, making &/or designing, I have never seen or specified one!
Badly driven rivets….have seen some horrors there; I remember seeing the first Viscount wing that came to Weybridge for re-sparring, double holes & bent rivets (nailed over, not formed correctly), it was apparently the first wing produced at Hurn.
As for riveting from the outside, yes done a few, usually because of lack of space on the inside to get a gun. Usually the head is formed using a domed headed ‘set’ on the gun.
Keith