My e-filing (after extension) was rejected, stating missing IP Pin, which I never received. Do I get 5 days to file without penalty, as is the case for Apr 15 filing?

You have until the 21st to either resubmit the return successfully or mail in the return with a copy of the rejection email.  https://www.irs.gov/identity-theft-fraud-scams/retrieve-your-ip-pin

To enter a 6 digit IP PIN

• Click on Federal Taxes (Personal using Home and Business)
• Click on Other Tax Situations
• Scroll down to Other Return Info
• On Identity Protection PIN, click the start button

Or enter ip pin in the Search box located in the upper right of the program screen. Click on Jump to ip pin