need to be able to do some minor editing on some of my pictures (rotate by a few degrees, crop, color adjustment...) that apparently are not possible in iPhoto.
These are possible in iPhoto.
This library has a number of low-res BMP screenshots (some are 16-color),
Not a supported format.
are the photos in the Aperture library accessible through the OS in the same way iPhoto's library is?
Yes, if you a: check the option to Share Previews with iLife in the Preferences, and b: generate Previews. Note that it's the previews that are shared, and pretty much in all the ways that the iPhoto Library is shared.