Like t3chb0t, I am not familiar with VBA, but I will mention a few things that caught my eye:

1) Lack of documentation. To me it is surprising, that you went through the trouble of documenting a class that is an event argument, but completely left out `IAutoComplete` interface, which plays a pivotal role in this entire thing. I can make an educated guess on what most of the members do but by no means it is obvious.

2) `config.UserSettings.AutoCompleteSettings.AutoCompletes.FirstOrDefault` - here you can use dictionary instead of doing a linear lookup.

3) `autoComplete.IsEnabled = setting.IsEnabled;` - you might want to consider replacing it with `autoComplete.ApplySettings(setting)`. Even though it "leaks" settings class to the interface, it makes a job of extending and/or customizing settings much easier.

4) Personally, I prefer to split `bool Execute` into `void Execute` and `bool CanExecute` (what Microsoft does in `ICommand` interface). I think it separates concerns better and allows to override those methods individually. It also makes it clear, that `bool` is not the result of operation (success/failure) but an indication of whether or not operation can be executed in the first place (can execute/cannot execute).

5) What pissed me off about Resharper is how hard it was to just input a single `{`. Without the `}`. And how it then removes both braces when you try to remove the second one... AFAIR they did something about it in later versions, but I am not sure what (maybe I just got used to it). You seem to mimic this behavior but I would love to see it improved in some way. :) Maybe add a setting, that can toggle this feature (the delete part) on or off. 

6) Another thought about `DeleteAroundCaret`: maybe `DELETE` should be handled internally by autocomplete classes? I mean if autocomplete is responsible for adding "output", it should also know how to remove it. At least it makes sense to me.

7) `VBENativeServices.KeyDown += HandleKeyDown;` - I think you need another abstraction layer on top of this event, that would track which keys are used where. From my experience handling key presses in hotkey-heavy apps quickly becomes a nightmare, if you use a single event for that. You should come up with a way to manage hotkeys and detect conflicts. When multiple services want to handle, say, `DELETE` key in different way, there should be a clear policy in place, that should decide which service should handle this key in the current context, or throw an exception if the hotkey is ambiguous.