I'd probably send one last reminder to the clinician. If they're written letters before, then they should know how important this is. Even with that disclaimer on the VIRMP site, I would still recommend being cautious because I know of someone whose application was nullified, waiting for a letter. They had to scramble outside of the match and ended up in Canada for a year. Don't get too stressed; you've done your part; now just make sure that your reference does theirs.