My take on Perception and Investigation: Perception is passive and broad. You just notice stuff. Investigation is active, your focus is much more. . . focused. You aren't going to walk into a room and notice the body in the corner was stabbed in the back seven times, has been dead for about 3 days, has $7 and pictures of his kids in his wallet, looks to be about 37, and was clutching a ring in his pocket. But your perception would allow you to see the body that was quickly hidden in the corner under a pile of dirty laundry.
So to me, you would perceive something, and then investigate it.
My take on Chaotic Good - Roz Al Ghoul (or however you spell Christian Bale's Batman's nemesis). He wants to fix society and all of its wrongs. And goes batshit crazy doing it. The ends (a repaired society) justify the means (destroying a city to make a point). So, maybe you do kill someone in the party (me) because I became evil and you are good. BD would probably join you.
Necro and good and evil. The handbook says something like "non-evil wizards don't use it very much", or "Only the evilest of wizards make regular use of it". I'm having trouble finding a Lawful/Evil example though.
As you said, I CAN use the dark arts without going bad! Just like I could handle using the ring in LotR without going Smeagle. IF using necro started turning me evil, the DM would have to enforce it somehow. . . without me knowing? My mind is being corrupt, etc. Not sure how this is done. I can only come up with Troy is telling me, and I have to RP that I don't know. I guess maybe by manipulating rolls by giving me +1 when doing bad things, but -1 when doing good? Somehow hurting those I care about forcing me to decide to keep doing the stuff (because it's also killing the monsters) or not hurting you guys any more? You guys keep ragging on me cause I'm desecrating graves or you cheer me on because I'm getting shit done? If you guys cheer me on, and you being sucked into evil too? This will be tricky and Troy will have to figure something out.
Do we declare our specialties as well? Do I let people know I'm going Necro? I'm guessing not. . . can be implied in our backstory or something. Because I can take the Necro spell without going Necro wizard, I just get some undead bonuses.
But over all, I'm a good guy. I just see the power of the dark side. And we'll have to RP the intervention and possible removal of a party member. If I go Sith Lord and you guys won't stand for it, then Alex will curse you all and make sure you know "You'll see!". And then I reroll another character, do some artificial leveling, we meet at the bar, and we carry on. Because, just between you and me, I do plan to go full necro, because I believe I can keep the powers in check.