Hey @ltsadmin welcome to the community.
For sharing passwords maybe you want to consider using a password manager?
How to deal with external client I have also written a post about this topic here.
It really depends on your set up as well and how they share it (if via a task or Asana messaging)
As Paul explained here you could set up a process that duplicates tasks and pulls info into a hidden project for you maybe as a backup.
Allowing to see comment edits is not possible atm, but you might want to upvote here: Allow to see and keep track of comments edits
Does that help?