If all of my 1256 option contracts were closed prior to year end (and within the calendar year), how do I enter them into TT SE during the interview phase?

you should be getting a tax form from the brokerage.  in the meantime, the net should be entered on form 6781.   this will split it 60% long-term 40% short-term flowing to schedule D.  when you get the 1099 or other reporting form from the brokerage check that the numbers agree. note that brokerages may not issue the forms until 3/16/2020.  so you may want to file beforehand if you're sure the numbers will agree.   otherwise, you may have to file an amended return.