Am testing adding a simple gallery to a blog post by including the shortcode from the [actual] gallery created. Seems like an extra step, but it resolves the problem with the lightbox not working properly.
It is webmaster’s intention to not bog the site down with additional and unnecessary bloatware (gallery plugins, Jetpack, et cetera).
Gallery is set to Basic Grid, and thumbnails are enabled to force the row height. After testing this seems to be the cleanest look. Lightbox functionality is not affected.
Images appear actual size, as they should. Made some minor changes to lightbox configuration. Arrow and esc keys work, as they should.