You can use any lens or focal length for landscape photography. Going from wide angle to telephoto, does provide a difference in perspective, but it also does something else. The wider angle (shorter focal lengths) brings in more foreground into the picture (pushes the landscape further into the background), while the longer focal lengths shoot over the foreground (excluding it) and emphasizes the background landscape - essentially pulls it in. So there are two distinctive styles of landscape photography right there.
I don't know if there is a "best" focal length for landscapes, or cityscapes - but its been observed quite a bit that the most popular focal lengths by professional (commercial) landscape photographers is in the mid 20's to mid 30's - or think around 28mm give or take a few mm. The reason why is distortion - or lack of distortion. 28s tend to be sufficiently wide (especially for full frame sensors) with out having to deal with the distortion that wider angle lenses bring to the table.
There are times that you want to go wider, however you do not want to deal with the distortion of wide angle lens. The solution here is to stitch. With stitching you can go as wide as desired, while not having to deal with distortions (specifically lens distortion). Another aspect of stitching is the addition of pixels to the scene. Stitching involves "stitching" several images together to form a single resulting image, with the resulting image being larger (in the number of pixels) than any one of the individual contributing images. Compare this against with the use of a wide angle lens for the same scene. The wide angle lens is going to "distort" the image of the scene, in pulling more scene in to the available pixel space. To put it another way, with a wide angle lens, each individual pixel will need to represent a larger area of the actual scene. With a longer lens, each pixel will represent a smaller amount of the overall scene, hence the more telephoto image will appear sharper with better resolution. Now to be fair, there are scenes that stitching just will not work, in particular scenes where things are moving.
An aspect that I have not touched on yet, is the sensor. The K5 / K5IIs has a wonderful sensor with additional sharpness with out the AA filter. So the better the lens you use the better the image. The K5's sensor has the same pixel pitch and density as the Nikon D800/e and the Luminous Landscaper noted this, that Nikon recommended the use of their best glass in order to bring out the best in the sensor. The same would go for the K5IIs....
In your post, you addressed corner sharpness of a lens. There are a couple of ways to address this. The first one is obvious, and that is to go find the best lens available with emphasis on corners. They tend to be expensive. Another approach is to use the combination of the K5's cropped sensor and a full frame lens. With a full frame lens, the image circle projected on the sensor is larger. That coupled with a smaller sensor, results in the lenses center falling on the cropped sensor. The result is shooting more through the center of the lens, and not the corners. So, in this respect, looking for and using older full frame lenses has a two fold advantage - using the higher IQ quality of the center of the lens, and the older lenses tend to be a bit cheaper.
An example of this is the 28mm lens. You can get a run of the mill 28 for $20. You can also find very good quality K, M, A and FA 28's anywhere from $60 to $250. Staying with the older lens concept, Carl Zeiss created the Contax line of cameras and lenses that are now discontinued. The lenses are superb and can be remounted to the Pentax K mount for 1/3 to 1/2 the cost of the Pentax 31 Ltd. I just walked through the lens database a bit selecting the older 28 to 35mm, 9+ rated lenses across the K, M, A, F older models as a sample. There is a lot out there....
When talking about landscape lenses, the one aspect that really does not come up is auto focusing. For the most part you are going to be at infinity, things are not moving, auto focusing is not necessary, and a manual focus lens really has few disadvantages. The only disadvantage with a K or M lens is the reporting of aperture. They will just default to a static reporting of f2.8 in the EXIF meta data.
In the end, the only thing that matters is what you want to do, and how you shoot, along with what lens appeals to you - and your checkbook (or your spouse's tolerance to your hobby).