Development
Unfortunately I believe you're blaming Corel for something that is not their fault, I just wrote an article for Signs and Digital Graphics Magazine about opening PDF files in applications that are not true PDF editors, Illustrator for example CorelDraw also. If you want to do reliable work you need to open the PDF in Acrobat, I use the PitStop Pro plugin. I identify fonts convert all to curves if the font allows it and then open in CorelDraw if edits are required we run tons of signs and wide format graphics from supplied PDF files. It's important to remember that Illustrator does not save true PDF files unless you properly publish them, which is not done by default, many fonts are not tagged for proper embedding in a PDF. If you want to take PDF files from clients you need professional level PDF editors, I know they cost quite a bit but screwing up jobs cost more.
One small extra observation: you are using X6. Have you tried X7?
I ask because PDF is a moving target -- there is no International standards organisation that defines it and could prevent adobe from constantly finding different ways of writing their PDF files to make PDFs hard for Corel to import. As such, PDF is a good way to send finished work that only needs printing, but a bad way to send work that requires further editing.
Corel does try to make reasonable efforts to keep up with changing ways of writing PDFs and consequently each release aims to be able to import PDFs a little better than the previous one. So, X7 may possibly import more PDFs correctly than X6. But there is no way the job will ever be completely done, and it is highly likely that there is a team in adobe whose sole purpose is to create PDFs that other companies find difficult to import. So, however much work Corel puts into it, there will always be PDFs from newer programs that CorelDraw cannot import.