MF is very very easy for landscapes. You can set the camera up for WYSIWYG (my preference) - it's all through the menus and 'Live View Setting Effect (ON I think)? Or is it OFF, I forget.
Then use the zoom function to quickly get in close for precise focus. Press once, focus box appears press again you zoom in, press again, even closer.
You get Live View on the rear screen and through the EVF, you just touch the shutter to deactivate the zooming in, or scroll around to your desired focus point.
I wouldn't bother with AF on non-proprietary lenses for Landscape use personally, it will be much slower/less satisfying, ymmv.
I would have thought having the L Plate (tripod mounted) on the Adapter, rather than the Camera Body, would be the least stressful solution, but I don't use lenses as big as 70-2/300 zooms. That way the adapter takes the weight of -> the lens or -> camera, rather than the big heavy lens -> pulling on the adapter -> pulling on the camera?