I agree with everything you said. If you want, you can write all the work/school excuses you want. However, I expect there may be a line of patients waiting for retail pharmacists to evaluate and counsel and then ask for a work/school excuse, esp in cold/flu season. (I know they do that already, but expect more esp if they know they can get a work excuse from you without having to go to their PCP)
In addition, that letter that Sparda wrote is now a legal document that states in his professional judgement, that patient is sick enough to be excused from jury duty. Hopefully he documented the encounter in a clinical progress note. If the patient clinically deteriorates (ie wasn't a viral GI bug but HUS, or acute cholecystitis, or ascending cholangitis, or pancreatitis, early DKA, or C diff colitis), then Sparda will need his progress note from the encounter documenting the pertinent negatives and positives on ROS and exam, as well as the counseling that he provided (eg., seek medical attention if the following occurs, etc). If the patient deteriorates, a lawyer will argue that the patient was harmed due to delayed diagnosis/intervention and that Sparda was negligent in timely referral OR missed warning signs OR inadequate counseling. If there is no progress note, then there is no documentation of anything that Sparda did and it will be a he said/she said situation. Once you start a progress note (for every sick excuse request), you will need to keep those records in accordance your state laws (duration, accessibility, kept HIPPA compliant, etc)
Your malpractice insurance might deny coverage (they might claim it is outside the defined scope of practice in the insurance contract) - or if they do cover it, may raise your premiums to cover the increase risks exposure. *scope of practice that the insurance agreed to cover you. Medmal won't cover a PCP doing neurosurgery unless the contract specifically agrees to it.
