If I settled on a house in DE in Dec 2018, but didn't move into the house until Jan 2019, do I need to file in DE for 2018? I lived/worked in MD for all of 2018.

No, you don't need to file for Delaware.  As you state in your question, you moved in January.  You will file a MD resident return for 2018, and a DE resident return for 2019.  (If you work outside of DE, you will file a Nonresident return for that state as well).