Mary Lincoln’s insanity was evident long before the death of her son Willie from what I’ve read over time.
Yes, she was “eccentric” from the time of their courtship and was NOT Lincoln’s first or true love
Have read that when their son died in the White House, Lincoln had to threaten to have her committed to St Elizabeth’s if she did not pull herself together
She also lost her brother to the Civil War, and supposedly suffered from hate mail from people cursing her and blaming her for the war dead
Spiritualism was very popular at the time and seances were common in the homes of the wealthy. It didn’t have the stigma it now holds. He seems to have joked around during the seances and had fun rather than take it seriously.