Trivial Trivia - taking trivia to new levels

[LincolnMan]

In March of 1865, President Lincoln had a meeting with three Union officers aboard the River Queen at City Point.

1) Who were the officers?
2) Name the artist who memorialized the conference in a painting
3) What is the name of the painting?
4) Which officer was, apparently, a "self-invited" guest to the conference?