generic interface and method with additional type constraints

Quite a lot of trickery indeed. I didn't quite follow throughout your entire post but I think the additional constraint issue can be solved with an extension method:

public interface IGridColumn<TItem, TElement> {
}

public static class IGridColumnExtensions {
    public static IGridColumn<TItem, TElement> Editable<TItem, TElement, TKey>(this IGridColumn<TItem, TElement> column // etc) where TItem : IHasId<TKey> {
        ...
    }
}