Users that have earned 10000 reputation can see deleted posts (and so can moderators, of course), but others can't. When a question or answer is deleted, the possibility to respond to comments is lost, which I admit can be problematic.
It is possible to get a question or answer undeleted, but in this case I find it unlikely. I have understood that posts will not be undeleted for the sole purpose of continuing a discussion in comments.
There is no way to contact a specific user directly. You should avoid sending them comments under unrelated posts (although that would be possible) and there is no private message system. With sufficient reputation (20 or 100) you can use the chat, but the discussion is public and you only reach those that use chat.
If someone has an email address in their profile, you can consider using it.
But do not use email for discussing deleted comments, closure votes or anything of that kind. Only send email if you know the recipient is actually willing to receive email from you. As a general rule, don't use email to discuss MO things.
If you are dissatisfied with the actions of a user, you can always contact a moderator. But please remember that not every issue is worth moderator intervention.