You can remove other attributes from the edit and reply forms by following below steps:-
1. Open EzEdit of qlistform View webpart.
2. Go to display field screen.
3. Select Content Type as “message” from Content Types dropdown, available at top of the display field screen.
4. Select display fields.
5. Save all configuration
We are analyzing your other issue “Is there a way for Discussion Subject to display when replying?”.
We will get back to you soon on this issue.