Welcome to the forum!
I'm sorry to hear about these problems. Fortunately, it sounds like it might be relatively simple options somewhere that you forgot about or accidentally set.
1. The PDF looks like it's blank, but you can select things, and see their bounding boxes?
My first guess about what's happening is that the entire layer's opacity has been reduced almost completely. Open Layers menu > Layers...., and look at the Opacity slider. Make sure it's 100%
2. If you're using the Pen/Bezier tool for everything in the file, it could be a Shape option is enabled. Look on the control bar for the Pen/Bezier tool for the Shape dropdown menu. Make sure it says None.
3. I probably need to look at a file which is showing this problem, to investigate about Export PNG. Are you also saving SVGs? Or do you only save PDFs? Since some things might not be editable as PDF, it's probably a good idea to always save an SVG file, just to be safe.
4. This sounds like a performance issue, which is usually caused by a very large file size. Have you ever used File menu > Clean Up Document (or earlier versions it was called Vacuum Defs). You can try that right away.
For everything else, if you could share a PDF file that is showing these problems, we can investigate without having to guess. If you have an SVG file from before you saved as PDF, please share that too. If there's no SVG file from before saving as PDF, then we'll just have to work with PDF.
Also, please tell us your Inkscape version and operating system.
