1) A texture is only loaded once, regardless of how many objects draw from it. Generally you don't save (much) memory from using a single texture, but you improve the loading time and rendering performance by drawing from as few texture atlases as possible rather than individual image file textures.
2) Yes. You can have, for example, RGB565 (no alpha), RGB5551 (bool alpha) or RGBA4444/RGBA8888 textures in the same scene, no problem.
3) Use Instruments. Check for leaks. If you're not using ARC, now is the time to switch. PS: don't purge caches, that's just wasting time AND memory because eventually textures need to be reloaded again. If you want to release a texture you're certain isn't in use at the time and not going to be used in the very near future, then uncache those textures and only those.