Hi @cmfly ,
I have made a test on my side for your issues.
For the first question, I’m afraid you can’t remove the ‘Subject’ column when use the Update item action, as the field is a default setting of discussion board list.
For the second question, you should use the first ‘Subject’, because just the first one has value, the others are empty.
Last question, please make sure the Trigger is about SharePoint. Only when the trigger is related to SharePoint, you can add a SharePoint list or library as owner.
Best Regards,
Zhongys