Electrolysis works by literally inserting a needle where you want to destroy the hair follicles. So you would only use it on the remaining hair, yes. It also hurts like hell compared to laser. and since they're zapping each hair individually it also takes a lot longer. Do laser first, if some hairs remain go for electrolysis.

Depends on the surgery. If it's for bottom surgery then yes, electrolysis will be necessary. Your surgeon should give you clear instructions about where and how they require hair removal.

It's for hairs whose coloration isn't dark enough to accumulate/absorb enough heat (70°C) from the laser in order for them to be incinerated. I'm currently doing belly, chest and face and - goddammit - I have two patches, one on each side of my face, just below my ears, that are white like snow... I'm going to have to do electrolysis ("diathermy" in my language). My threshold for pain is so low, that I need to apply morphine cream on my face at least half an hour before she zaps my face, so I can't even image what the electrolysis is going to feel like 😅 as long as she does it, I don't mind though. I'm not a masochist, I just adore her. 🩷