Keyboard shortcut within Mission Control for choosing a specific application window?
If you REALLY use Mission Control a lot there IS a workaround that must be configured individually:  

[EDIT:] I wrote an "elaborate" Karabiner Elements script (json); it's linked at the bottom of this post.

The very commendable 3rd party software "Karabiner Elements" (interim version) allows to simulate mouse movements in a "general" way, meaning: no exact positioning to screen coordinates, but just movements to left/right/bottom/top (or combinations of these).

Moreover its "mouse moves" depend on your System-Preferences' mouse velocity settings.
But still, if you invest a little time for finding ideal values for your own mouse, you'll be rewarded by a shortcut "script" that allows to (e.g.:) move between visible apps' windows through arrow-key-directing.

A typical code particle looks like this:
{"mouse_key": {"x": 3072} which moves a mouse cursor "fast" to right, "y" values moving up/down ("3072" = max. value; depending on your mouse settings one click would "call" this code several times). 

Here arrow keys move cursor, return/enter take a selected window back to your space.

An over-simplified "script" would look like this:

{"title": "Mission control, navigation",
 "rules":[
            {"description": "Mission control, navigation",
             "manipulators": [
                   { "from": { "key_code": "right_arrow"},
                       "to": [ {"mouse_key": {"x": 3072}} ] },
                   { "from": { "key_code": "left_arrow"},
                       "to": [ {"mouse_key": {"x": -3072}} ] },
                   { "from": { "key_code": "down_arrow"},
                       "to": [ {"mouse_key": {"y": 3072}} ] },
                   { "from": { "key_code": "up_arrow"},
                       "to": [ {"mouse_key": {"y": -3072}} ] },
                   { "from": { "key_code": "return_or_enter"},
                       "to": [ {"pointing_button": "button1"] },
                     "type": "basic"     } ]
}        ]  }


As mentioned: this code lacks essential ingredients, is even dangerous:
E.g., in every app you use a "return" or "enter" would be replaced by a mouse click!

So firstly: it is NOT restricted to the "Mission Control" view; reason is: there is no actual app to aim at! … MC's "frontmost_application" is the top one in "normal" view, e.g. Finder or Safari.
This MUST be handled by calling "mandatory" modifiers (I recommend/use: Control &amp; Shift).  

Next: above, you have to keep arrow keys pressed while the cursor "rolls" across your screen.
This is improved by making "multiple" (= farther) "jumps" in one key press AND preventing key repetitions.
About 4 to 8 "moves" (= times "arrow" pressed) to get from one end of your screen to the other seems practical to me; the return key will select an app's window and "go back".

I installed a much more elaborated code version on my MacBook-Air – and it works quite nicely.
Maybe, if somebody's interested, I'll post a json file here, once I optimised  all facets of it …
Here is my code for a "realistic" mouse jump behaviour:

https://gist.github.com/clemsam/ef03ed5127fd9783267b60065c70fc17