I read your post, examined the photos, and initially failed to notice the small detail you referred to. By blowing up photos 5 & 6 and comparing the right side of photo 5 to the left side of photo 6, I now see what you’re referring to.
The angular narrowing of the left side of the projection might be explained in one of two ways.
1. Something is masking the right side of the negative to project the angled side as seen at the left of photo 6. This seems unlikely, but should be considered.
2. If no such masking is present, then the lens axis is skewed relative to the baseboard so that the projection is distorted. Essentially, the projection distance is greater at the upper left corner than at the lower left corner [note: I exchanged "upper" and "lower" in the sentence to correct the statement from its original form]. That gives a varying magnification of the image. Closer areas are magnified less. Farther areas are magnified more.
The small amount of adjustment obtained by adding tape to the contact pads of the easel is too small to account for the problem as shown. Since you’ve adjusted the easel so that its top surface is parallel to the baseboard, the problem is clearly not connected to the easel.
Given the depth of focus above the image plane, you can make a crisp, well-defined print as is. If you want to eliminate the angular distortion (too small in this case to matter for most photos), you’ll have to adjust whatever is causing the misalignment between the lens axis and the baseboard.