You can, but the only way to do it is feeding vitamins. Since vitamins max out at 100 EVs, you won't be able to get more EVs than that into any stat afterwards.
If you use an EV reducing berry in DP and you have 111+ EVs in that stat, the berry will return you to 100 EVs. If you have 110 or less EVs in a stat, the berries reduce your EVs by 10 each use. So, if I had 252 EVs in Attack on my Ursaring and I used an Attack EV reducing berry on it, it would have 100 EVs in Attack afterwards.
Going by the list in the research thread it looks like Grass Knot and Stealth Rock are one time only. Anyone know if it's possible to buy/win either of these in DP?
They're Gym Leader TMs, so tough luck here. I'm pretty sure you can't buy them in PBR, either, so you'll have to restart your game to get more of any of the Gym Leader's TMs. Of course, there are a few that are learned by Pokemon outright (that can be passed on to other species), like Shadow Claw, Flash Cannon and Brine. Electrode learnes Charge Beam, too.
Whats up with those little personality descriptions you see when you're looking at the sheet with your pokemon's nature on it? Like "often dozes off" or "likes to thrash about"?
No one's 100% on these yet, but they definitely describe an IV. Whether or not it's your highest IV or it's just some IV-based flavor text (no practical value) is still up for debate. ADH made a thread on 'em.
I know the Gible question was answered earlier, but is there any requirements I have to complete before he appears in Wayward cave, I helped that girl out of the cave, and I looked in the Morning, but no luck, I have yet to check in the day if the day starts at Noon though, as I had to stop checking at 11:30 or so on account of school, so, where is he?
He's not in the same cave as the girl. There's a secret entrance under the bridge to the left of the main entrance that opens up once you rescue her. You need strength to go further. Once you're in, make your way to the basement, and that is where Gible is; he's not on the floor you enter on, to be clear.
EDIT
I changed the clock to Friday, and am in the grass outside the Windworks, but no Drifloom =[.
If you change the DS internal clock, you have to wait 24 hours from the time your game realises this until time-specific events will open back up to you, i.e. Pal Park, Drifloon, Rival Battles...