Well, as for your first question, I'm not surprised that you don't have lsof, it is not standard. I do not know of a version for AIX.
As for your 2nd question, Unix does not maintain a list of users who accessed stuff. You can tell that that some process accessed a file because atime will generally change. See my article,
mtime, ctime, and atime for more info on that.
Maybe you can consider another OS. HP-UX can be converted into a trusted system and then full auditing can be turned on. Auditing records every system call. This requires a lot of disk space and is a heavy performance hit. But you can then inspect the logs to see who accessed which file. I believe that Solaris has something similiar, but I have not used it. And you can get lsof for both Solaris and HP-UX from the freeware sites. There may be auditing features available for AIX and there may be some way to get lsof for AIX, but I don't know AIX very well. Maybe some AIX expert can add some more info.