Yes, you can be denied entry again. I was denied entry twice back to back. The next time I attempted to enter I had solid ties to Canada. It sounds like you may be lacking ties to Canada. I suggest waiting at least 6 months before trying again. Use that time to form ties to Canada.
We, as Canadians, often take for granted our easy travel to the US.
Mail the withdrawal letter after he leaves the US.
If you follow the aforementioned advice then there won't be any overstay (I'm assuming that Form I-485 was properly filed before his I-94 expired) https://fam.state.gov/FAM/09FAM/09FAM030211.html
I'm very sorry to hear about your parent. Have you tried expediting your I-131 Advance Parole application? Your situation qualifies under "Emergencies and Urgent Humanitarian Reasons".
My reco is to contact your local Member of Congress (who is up for re-election if possible) to expedite. Go to the member of congress' website and look for "help with a federal agency". Fill out the web form/privacy release form and someone will contact you within 24 hrs. You can attach proof of the emergency etc.
I am speaking from experience expediting my case. If you expedite via USCIS, you will be waiting forever. My case only started to move when I seek the help of my senator to expedite. Just a note that it is important to know that your Senator or Congressman/woman cannot influence USCIS but can only advocate on your behalf. The benefit of contacting them is they have access to connections you would not have if you had just expedited yourself (ie Member of Congress' office has access to Liaison Office).
Good luck!
https://www.uscis.gov/forms/filing-guidance/how-to-make-an-expedite-request#:~:text=You can generally request expedited,have obtained a receipt notice.
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