If I activate the 'One Hit Kill' and 'Infinite Ammo' tweaks, can I still get all of the trophies that I would without them, like the '-insert number here- kills with -insert weapon name here-' trophies?
I wouldn't. Usernames are supposed to symbolize you in a way that your real name can't. For example, your username is 'gigastrike', possibly meaning that you like to be powerful. But if your name was, for example, Steve, It wouldn't exactly screwam "ULTIMATE POWER".
*sigh* I'm so tired of people asking about other peoples opinions on whether or not to buy a game. If the game looks good and you want it, buy it. If you don't want it, don't buy it. If you're not sure, rent it first. It's that simple.