Unfortunately, we have no plans to support older versions of IntelliJ, because it requires significant continuous efforts.
Also, the WYSIWYG UI designer is web-based, hence requires JCEF provided by IJ to run. However, we could make the other parts of the designer (Component Inspector and other panels) work without the canvas if JCEF is not available. It will make sense when Projector is updated to the latest versions of IJ.