I see you managed the encryption after all. That makes me happy.

Users can decode mesg on steemitchat with steem keychain (Private posting key)

Is there a reason why it has to be the private posting key? I would prefer the private memo key for these purposes.

Is this your operation structur?

required_posting_auths : [ "from_user" (stmpak.wit) ],
id : "steemchat.msg",
json : { to : "user name who receives the msg (random.user)",
text : "message text"
}

The screenshots from Steemworld and WhaleVault show a different structure. Have you changed it in the meantime?
I find id: "steemchat.msg better than Private Message ... A short and clear id makes processing easier.
And I think the fields from and to in the json data are a good solution.