Thanks for taking the time to provide your feedback.
I discussed this with the development team and I can advise that you can write your own messages to be displayed for a given event using the new TriggerNotificationEvent API . This has now been added to our developer documentation.
This way you can create a custom notification or custom message for any event which has an action hook point. One would create an action hook script to call the localAPI to TriggerNotificationEvent and write your own messages in the attributes.
So for example, in situation 1, you'd use the ClientAdd hook point to call the TriggerNotificationEvent command using the localAPI, passing in whichever description you wish to display in Slack.
It will be passed to Slack/Hipchat to handle.

Please submit requests for new notification points separately.