I agree with the decision to pull the scene out.
Having the baby onscreen also clearly infers that Marla's death was very recent. Without the child on board, we might assume that Marla died over 14 years ago, and Khan had all that time to go insane.
As for unused footage, the "child's face in the window" has the added problem of Nick Meyer improvising, and getting the only available (white) stunt person on duty that day to do the scene - in blackface makeup
- of Terrell rolling down the hill, having received a fright (ie. the stunt fall wasn't scripted).
When I did an interview with Paul Winfield, just before the film came out, he said that the production was reported to the Stunt Actors Guild for that breach of protocol. As in, it was disadvantaging opportunities for African American stunt workers. So we are unlikely ever to see that scene.
Re the painting. I've never seen an image on the 'Net, but it was definitely created. Judson Scott described it to us at the convention. (You'll recall "Space Seed" established Marla as a portrait painter.)