well, as for #1 straightening would be easiest in photoshop. I'm sure it's probably possible with elements though I'm not exactly sure how you would manage it. I will say when you take photos like that if you're able to hold the camera parallel (or I suppose perpindicular (sp?) to the item being photographed you'll have a better time keeping the subject with straighter lines.
2. I would probably just crop that area out then, easy enough to do and you don't have the distracting vignetting. I mean, if the vignetting were to be even all the way around it would be ok, but since it's awkwardly shaped I'd probably try to get rid of it.
5. I know that the tokina is a 2.8 lens, however that merely means that the maximum aperture (or the smallest number) throughout the range is 2.8. If you want a larger DOF (which is almost never a problem with wide angle lenses because of how DOF works) you can just use a different aperture such as 5.6 or 8 (lenses are usually sharper stopped down a bit anyway)