Since I don't have much time for further plugin developement and new Sketch releases produce bugs pretty often I decided to discontinue the project support. Sorry.
Update: All sources are now available in this repo. Enjoy!
Keys installation is way the same as any other plugins. Just download latest release and double-click it.
...on the right side
The only you can do is check the issues list and report your issue if no-one did it before. Right now Keys supports only the latest stable Sketch version, so you should expect an update right after its release.
On the Releases page.
Use «Restore Default Shortcuts...» command from plugin menu. It should remove any user shortcut data includinge one that defined directly in System Preferences.
Since Keys is using default system storage for shortcuts it shouldn't affect previously set shortcuts without additional user actions like «Restore Default Shortcuts...» command from plugin menu.
Since System Preferences caches shortcut values from storage once upon a startup you need to completely relaunch it to get updated shortcut data there.
Nothing, because all your shortcuts were defined in System Preferences.
Since Sketch manages single character shortcuts by itself, you should delete
~/Library/Application Support/com.bohemiancoding.sketch3/ folder and restart Sketch to return default tools shortcuts.
Internally Sketch may not properly draw some menu items prior to their first appearance in menu, that's why the only way to make these kind of menu items configurable with Keys is to manually access it through the app menu first.
Unfortunately, there isn't. In simple terms, there's no capability between different Swift versions right now so it may produce an errors at runtime.
But it's not really a big deal. Firstly, the Swift team has already announced that ABI capability feature is planned for the next major Swift release and this is the first priority right now. Secondly, plugins can't conflict each other while the latest available Swift version is used for all of them and requesting author for an update may be a good temporary solution for this problem.
Since Keys plugin is wrapping a default System Preferences shortcut assignment flow it follows the same rules. So any default shortcut are constant until you either replace it with your own or use the same combination for another menu item.
Since Sketch itself is using an internal predefined list of menu items that allowed a single-character shortcut to be set, Keys can customize single-character shortcuts only for these items.
Please check the open issues list and feel free to create a new one if you don't see your problem there.
I spent a lot of time to make this project real so any feedback or donation is really matters. It's wonderful to see that things you do are really making people's lives easier.