1. First of all, EV stat bonuses aren't allocated quite like that. For every 4 EV points you have in any given stat, your Pokemon will have 1 extra point in that stat upon reaching Lv. 100. If your Pokemon is instead at Lv. 50, then 8 EVs result in a 1 point bonus in your stats. You get the idea.
There are two reasons for a Pokemon's stats to increase when it levels up. The first is that the EVs it gained between the previous level and the new level can finally take effect. (Say your Pokemon hits Lv. 50... you finally get the bonus from all EVs earned between Lv. 49 and Lv. 50, unless you've used the Box Trick beforehand.) The second is the simple fact that it's getting closer to level 100, so its base stats increase and the EVs it gained previously have more of an effect.
2. All Pokemon who participate in battle gain the full benefit of EVs. If a Pokemon gives 1 EV in battle, and you use a full party of six Pokemon to defeat it, all six Pokemon gain that 1 EV.