your theory is realistic
There were a lot of cases before my PR (in 2015) when people passed second medical check.
For me it took 6 months between AOR2 and Medical Request (they did nothing during that time
). Background check was started after MR. Six weeks after medical passed I got passport request. In total PR process was 10 months (now a real processing time for the same program is 17-18 months)
I think, that visa offices processed Family applicants separately, because on IRCC web sites duration for these categories differs and we will get soon our BR, MR, RPRF, COPR and other "welcome to Canada" abbreviations