My daughter has chosen fabrics for me to make a duvet cover for her comforter. Being afraid to use lots of seams for teensy blocks, the cover will have 15 inch squares.

Question 1: What kind of thread would be best?
Question 2: What number should I set the stitch length?
Question 3: Would you use larger than a 1/4 inch seam?

I know she wanted a duvet cover rather than a quilt because she wants to be able to wash it in her washing machine.

Any suggestions would be so appreciated, and thanks in advance.