All evidence seems to point to Dave as being the murderer.
Maybe he thought keeping the house in a state of ‘permanent postponement’ protected him from making decisions to marry this lady. Maybe he thought she did not love him, and would leave him as soon as the project was done. Who knows? Perhaps he, Dave doesn’t even know.
Speaking of “Zany” stories about home remodeling;
This reminds me of the famed Winchester House near San Jose, Ca. The widow of Mr. Winchester (the gunsmith) thought if she ever stopped adding onto her home, the ghosts of everyone shot dead by her husband’s rifles would come to haunt her.
He did marry her. In 2017.
Except they were married.