You can't tell in this poor reproduction, but there are seven feathers sticking out of Papageno's head; one of them doesn't stand out. That is the same number as in Giotto's Stultitia (Folly), which the engraver surely would have known from engravings. It is also the probably the same as in the PMB card (whatever it was called), which has 6 feathers visible (but one more hidden per Kaplan), as Lorredan pointed out recently at viewtopic.php?f=12&t=889&start=10#p12977. It is possible that the Mozarts, as big tarock fans, saw this card or one like it when they were playing for the aristocracy in Milan (Jan-March of 1770, per Hildesheimer p. 380). If Papageno were a tarot card, of course, he would be the Fool. According to Moakley, surely correctly, the seven feathers are the seven weeks of Lent. Hence the sacredness of 7.
In this illustration, his fingers point in two directions, his middle finger to the temple in the background, his index finger straight ahead. It seems to me that they make a Y, perhaps the famous "Pythagorean Y", i.e. the choice of Hercules (or Scipio, in his dream; Mozart actually wrote a youthful opera on this theme: http://en.wikipedia.org/wiki/Il_sogno_di_Scipione).
According to Van Den Berk, the different fingers had significance in Masonry (pp. 330f):
To this I would add that Saturn himself represented Wisdom, and had since at least the time of Ficino. Also, if (as I say) the fingers represent the choice of Hercules, then the index finger points the way to pleasure and luxury, as in that allegory. However it is sometimes by following pleasure that one goes toward wisdom, too, if done with the proper restraint, as Plato famously, or infamously, argued in the Phaedrus; that is also implied in Jupiter's association with religion. Van Den Berk does not document these associations; but if the part about Saturn and Jupiter is right, their associations are well grounded in ancient, medieval, and Renaissance tradition.The middle finger represented Saturn, and its phalanges represent: economics, the sciences and philosophy. We know that "philosophy" means "love of wisdom". So we can understand why this finger points at the "temple of wisdom", the place of the highest science, that of the realization of the opus magnum. The philosophers' stone lies hidden in the dark lead of Saturn. The "divine economy" consisted in extracting the gold from this lead. The index finger stood for Jupiter, and its phalanges represent: pleasure, luxury and religion. Jupiter is the planet of the day time, of light and things spiritual. It undoubtedly points out the location where the celebration of the sacred wedding takes place.
On the significance of the number 18, Van Den Berk gives the following references: Landon, Mozart's Last Year 1791, London 1988, p. 128 and p. 228 note 9; Rosenberg, W.A. Mozart. Der Verborgene Abgrund, Zurich 1976, p. 65; and something by a Grattan-Guinness, p. 219, not listed in Van Den Berk's bibliography. A great merit of Van Den Berk's book (in English translaton) is its awareness of the German-language literature. However that doesn't make it easy for me to find his references. In this case, the main one is in English, so probably not hard for me to get.