Lots of women drive bare footed as they may have been wearing high heels, claiming it stops you apply full pressure pedals is daft no reason why it should. Totally depends on the car
Totally depends on the car and it's level of servo assistance. I have driven bare footed loads of times, despite not being supposed to, but I fairly regularly walk around on rocks bare-footed at the beach and my daily driver has over-servo'd brakes so it's not a problem to emergency stop for me. In my other car where the brakes are significantly less assisted, or in older cars where there is no assistance, a girly girl driving bare footed trying to get the car to stop would undoubtedly squeal with the pain of stamping on a 1" wide metal pedal in delicate bare feet.
As for flip flops – they're not exactly secure, but they do seem to stick to some peoples feet fairly well. I was never someone who got on with them, they always fall off and rattle around, it'd be stupid for me to drive in them.