I can answer a bunch of your questions with one thing. Every 4 EV points give one stat point. That's 63 points in all for ANY pokemon and any stat. Natures, however, give more stats.
1: Look up there ^^
2. Look up there ^^
3. as I said earlier, the margin is 63 stat points. So whichever; probably HP, but only if you can predict the physical hits from the special ones. Otherwise, Sp. Defense would be the way to go.
4. 252 HP takes hits pretty well. But let me explain. Let's say you have... a Mew. (same stats all-around). And you know you'll get hit with a physical move. 252 Defense will help more than 252 HP. But, 252 HP covers both sides of the spectrum, so... it really depends on your prediction. Though I'd recommend 128 HP, 64 Def, 64 Sp. Def for most standard.
5. Well, there are other factors. Will you be getting hit with both spectrum moves for its tier (I don't know that tier), or special/physical? Will you have Lefties or healing moves, or Cosmic power/amnesia/iron defense? There are so many factors, but if you have the formers, HP, the latters, Defenses.
Hope I helped! :D