Any way to find built in or custom properties in an Aspose.Slides.Pptx.PresentationEx object?
PPTX built-in / custom properties are not available yet. But we will investigate and add this feature as soon as possible and let you know.
Is there any way to open a .pptx file as an Aspose.Slides.Presentation type of object?
No, PPTX is completely different format and you cannot type cast PresentationEx object to Presentation.