Very nice theme! A few things I noticed:
1) Not sure how I feel about the three dot toggle in addition to to the hamburger menu on mobile. Would it be possible to combine these into one?
Not quite the same, but the Eighties theme does a good job combining widgets into a single toggled screen. You'll probably have to use jQuery to move stuff around in Montage though.
2) On mobile widths, the index/archive posts associated with short images are causing spacing and overflow issues.
Since the whole theme revolves around having variable height images, one way to handle this would be to simply display the text below the image when everything breaks to one column, even though you're kinda compromising the "desktop experience" to make it work.
That or limiting the amount of text by setting a max-height (maybe combined with dotdotdot) to make sure it doesn't overflow past a certain point, but that may look weird on the posts with tall images, and would have to be careful a "double ellipsis" isn't displayed if using dotdotdot.
3) Everything else, like the primary nav menu, single posts, comments, typography, etc. all look solid to me.
For accessibility, and just general experience, I think it would be best to keep the outline as-is.
This. You'll notice there's a "double scrollbar" when the menu overlay is active. One for the overlay, and one for the page beneath it. This can be solved with a carefully placed