A very interesting and well-presented discussion! According to my own unpublished research, at the beginning of 220 AD a star was placed in the reverse field of all Rome-mint coins of Elagabalus, including the coins he struck for his mother, grandmother, and successive wives. In the course of 221, however, four emperor-sacrificing reverse types were introduced for Elagabalus, and the so-called horn was added to his obverse portrait. From now on the star continued to be placed in the reverse field of all of Elagabalus' coins, but it was omitted from the types that he produced for his family members, namely Julia Soaemias (type VENVS CAELESTIS seated), Julia Maesa (type PVDICITIA seated), Aquilia Severa (2nd marriage, type LAETITIA standing), and Severus Alexander as Caesar (various types). This restriction of the star to the coins of Elagabalus himself after mid-221 fits well with Herodian's account that towards the end of the reign Julia Maesa persuaded Elagabalus to concentrate on his duties as high priest of his Emesan sun god, and to accordingly adopt his cousin Severus Alexander and make him Caesar, so that he could take over for Elagabalus the business of ruling the empire. Soon after the introduction of Elagabalus' four emperor-sacrificing types in mid-221, however, quite a few denarius reverse dies of each type were mistakenly engraved with the star placed behind the emperor, so had to be corrected by eradicating those misplaced stars from the dies and engraving new stars in what was obviously understood to be the proper position, that is in front of the sacrificing emperor. You show in your discussion above one such denarius with a partially eradicated second star behind the emperor; failure to entirely eradicate the second star doubtless stands behind Cohen's description of some such coins as showing two stars. These corrections would appear to be very strong evidence that the stars on Elagabalus' coins were meant to represent not the comet which had been seen in 218, but rather the sun god to whom the coin types of 221-2 depicted the emperor sacrificing, and whose image should therefore be placed before rather than behind him. I admit however that I don't know how to explain the tails that are sometimes added to the stars in front of the emperor, which could be taken as indicating that they represent comets.