Grid displays weirdly in editor

Two questions…

Problem 1:
The grid element on my site sometimes adds an extra column in dev mode that disappears in preview. Why is this? I’ve ran into this a few times.

Problem 2:
I’d like to make the BG video to span 2 columns instead of three. Where do I grab the element if I now want to change the span, to two columns for example?

Link to page:

Here is how you do it.