After the medical is passed, a decision often follows within a few weeks, commonly inside about six, but it varies from person to person and from office to office. A medical furtherance like a repeat X-ray adds time on its own, which is most of why this one has run past 20 weeks.
On your second question, IRCC does not publish how the stages run inside an application, so nobody can say for sure whether background checks continued while the medical was outstanding. In practice the different checks are handled separately, and a file does not simply stop while one of them is open, but there is no way to confirm that for a specific case.
What you can do: check the processing time for your permit type and country on canada.ca, and watch the messages section of your IRCC account rather than the tracker. If the published processing time has passed by a wide margin, a web-form enquiry is reasonable.
This is community experience, not legal advice. For your own file, speak to a licensed consultant or lawyer.