Since stealth rolls count as a block against detection it means that unless someone passes an alertness/investigation check the PC can basically do anything he likes without fear of being caught. Part of that is because he's rolling better than me (you have no idea how many vampires have died to crappy rolls in this game) and part of it is because of his combat-focused character (he has pretty much 0 social skills and he's the type of gamer that will opt out of a social situation via throwing minions at the opponent... and breaking her spine).
So he kills everything from the shadows and his stealth block stays intact the whole time? I am always going off the assumption, that you are becoming visible, when you attack someone. It is a common enough trope, and it just makes sense, if your target is standing in the light. Especially, since a block usually lasts exactly one exchange if the character doesn't renew it.
If there are two or more targets, he can't kill them all with one strike, and the other ones will surely notice what is happening. I would have him drop the stealth block for an attack, and if he wants to retreat into the shadows, he'd have to reroll the stealth block in the next exchange. While the opponents have all the time to act, and he'd only have his athletics to dodge, not the block. And even if he returns into the shadows, they know he is there. They would no longer be surprised by his attacks. They could do some sort of scene attack, throw a grenade, cover the scene with fire, things like that. People do crazy things when they are frightened. Those wouldn't have to be attacks, btw. A hail of bullets as a block or a grenade as a maneuver would be just as viable in this case. Light will kill the shadows, as I said before, and he just would not be able to do a stealth roll in the first place. Don't let your NPCs act too dumb.
Usually, you'd sneak to avoid being seen, not to kill everything. Killing people will be noticed sooner or later, and whereever he is sneaking is at heightened alert, he'll have a pretty hard time.
The Sight doesn't overcome blocks vs perception, it bypasses them.
That seems strange to me. If you have a way to overcome the block, you've overcome the block. In this case, the block would still be active, so other people wouldn't be able to see you, but the wizard would be ready to rain hell down on you. See Harry vs. Ariana.
You don't get to overcome a ward against tresspassing just because you can whistle.
That doesn't make sense at all. It would be more like being invited to overcome the ward. You are no longer affected by it, others certainly will be.