Extension tubes won't save you, unless you like wide-angle bugs
Maybe you could pull off a portrait with it at 20mm, I think the thinnest extension tube around is the kenko 10mm.
But you're definitely not getting a landscape out of it, and it'll vignette at the wide angles anyway.
Thought about the Sigma 12-24 versions?
Mk1 is almost perfectly rectilinear the whole range, not the sharpest lens in the world, although it's not bad, and CA is rather high.
Mk2 is rather sharp in the centres, rather soft on the wide-angle corners, field curvature is a problem but if you MF you can get around it. Barrel distortion at 12mm is bordering on fisheye though. CA about the same as the Mk1.
Best part is their prices, $830 or $950 at B+H, a lot less used.
What's wrong with the 16-35 II? It's not the sharpest on wide-open corners, and vignetting isn't the best, but stopping down to f/8 for landscapes both of that disappears. 17-40 ditto, soft corners aren't so bad by f/8 (although they could both be better for the price).
If you're not afraid of fiddling around, the Nikon 14-24 G is, well, very nice. No aperture ring so you're either stuck wide-open (on that lens, not a bad place to be stuck), or you can get adapters that set the aperture. It'll set you back a pretty penny, almost as much as the EF 14 f/2.8, but wider and better IQ than the two canon zooms.
Thought about other primes?
Samyang 14mm is a nice buy, especially with that price-tag, as long as you keep straight-lines out of the picture, fields and hills should be fine. 21mm Zeiss Ditagon will cost you more than most L zooms, but from what I hear it's worth it. Not much else in the 'ultrawide' range unfortunately, at least, not that's any better than either canon L zoom.