Can a PCT application be filed on a granted USPTO patent with a priority date that is less than a year ago? And if so, do national stage patents that issue from the PCT application get the same priority date as the granted USPTO patent?
Yes - The key issue is "with a priority date that is less than a year ago". The date of the grant is a non-issue.
As mentioned in a comment by OP @Keir Finlow Bates a US patent can issue very quickly. It could be a Track I or moved to the front of the queue due to age or ill health. And Yes, they get the same priority date as the US application. As @RishiM_IPR points out, priority comes from filing dates, not grant dates.