unfortunately parented window won't really help.
With the latest changes, keyboard handling event has been moved from each control processing, to NSApplication / NSWindow processing.
That helps to simplify handling of keyboard events. LCL doesn't have to override the method of every class it uses, but can keep all the code in one (two) places.
The downside for your project, is that you are loosing all the keyboard. Neither NSApplication, nor NSWindow are based upon LCL code, so nothing it transferred back to your controls.
It might be possible to make the latest version work for you, if keyboard event handling is done via global event monitors. But it still needs to be implemented.
I'm afraid, that the best option for you, is to stay on previous version