Yes, you can walk around on the roof--it will hold you. I typically try to put my weight only on the rivet lines, but this topic has been hashed over many times, and many folks offer the assurance that they step between the rivets all the time without issue. Just do not put any weight on the formed end-caps. They dent quite easily.
As for getting up there, I find leaning a ladder against the awning rail does the trick. If you are really going to do a bunch of work up there (ie., swapping out an AC unit), then renting some scaffolding to have a platform to work from is never a bad idea.
Once you find your leak...well it all depends on the nature of the leak, but sealants such as Parbond or Vulkem are usually the thing to use.