It's literally the most management problem imaginable.
Management decides the process, management decides who to hire, management assigns the roles and management decides how urgent the work is.
If the constellation is so that reviews are required, but the reviewer is not under the control by the manager and reviews do not happen, then some manager somewhere made a mistake.
Why? Because PRs are expected to be reviewed. Any process in which that does not work automatically is dysfunctional and needs to be fixed until PRs are being reviewed properly again.
If everything is working as intended, but it's the manager who created a false sense of urgency and pushed all of the responsibility onto the person who cannot do anything about that, then the only person who can fix the problem is the manager and the thing he has to do is something very easy: be more patient.
Edit: It might not be obvious, but all the things you claim that strengthen your position are already table stakes and assumed to happen regardless. The question is, who is responsible for making the invisible and automatic happen? And you're saying the manager can shirk his responsibility onto the blocked employee, which is hilarious because it means your entire operational strategy for fixing a broken process relies purely on being lucky.
Another thing that might not be obvious but if you write a bad pull request and the reviewer tells you it is bad and can't be merged that's on you as well, no shit Sherlock.