It’s not the case that you cannot use the Control key on Mac in general. But what you will find is that key combinations that are “dead”, e.g. that don’t produce an actual character but rather set a particular state in advance of the next character to be input, such as combining diacritics, cannot be used, because the Qt application framework’s system for key commands works on the produced character and not on the key codes of the keys that were pressed. This is a compromise that allows the key commands to be cross-platform, which is an important design goal for the developers of Qt. There are also some subtle bugs in this area, a number of which we have reported to Qt ourselves and some of which we have fixed ourselves. These issues are further compounded by the differences in keyboards in different locales, which mean that the same combinations of physical keys can produce very different results.
None of this is ideal, and in practical terms all you can really do for the time being is employ a bit of trial and error to find which combinations can be assigned reliably. I hope that over time the bugs in Qt will be ironed out, though we are somewhat resistant to spending a great deal of our own limited development time working on these issues ourselves because the point of using an application framework like Qt is to allow you to focus on delivering more valuable functionality to your end users, so we are pinning our hopes (for now) on some of these issues being addressed by their developers.