On Devauchelle's : how would one write Caesar in demotic? How different would it be from Djeser?
It's difficult to say. Demotic is an everyday cursive writing no longer bound to the hieroglyphic script as closely as hieratic is. You must read it from right to left. It is very difficult to decipher, even for skilled scholars and always subject to interpretation. See here for Caesar and Djeser. I'd like to be more explicite but although I teach ancient egyptian in french it's rather difficult for me in english ;-)
I'l ask my friends egyptologists for other occurrences of demotic writings of Caesarion's and Djeser's names.

And what does "year 5" mean? Five counted since what?
The ancient Egyptians counted years from the first regnal year of the living king. When he died, the calendar was reinitialized. This stela has been written during the fifth year of a Ptolemy (neither Cleopatra nor Caesarion according to Devauchelle), the day of the festival of Isis, or the birthday of ... Djeser (if Devauchelle is right, neither Caesar nor Caesarion were born at that time).

although if he was Caesar's son, would he have been engaged to Julia as Munzer proposes so plausibly? quoting Safiria and Asterix : Sono Pazzi Questi Romani ! :-) but if it were so... This (including the tragical conclusion in march -44) is a very good stuff for a novelist !
According to Thomas W. Africa (The Mask of an Assassin : a psychohistorical study of M. Junius Brutus, Journal of interdisciplinary History, Vol 8, n° 4 (spring 1978), p. 609), Quintus Servilius Caepio, to whom Julia was bethroted, was Marcus Brutus : "...His other uncle, Q. Servilius Caepio has adopted Brutus, whose name was officially Q. Caepio Brutus. [...] In 59 the first triumvirate was in power, and Caesar's young daughter, Julia was bethroted to someone called Servilius Caepio. Weighty authorities have assumed that this Caepio was young Brutus , and subsequent events make the conjecture plausible." It's rather complicated especially if you follow Sir Ronald Syme (Bastards in the Roman Aristocracy, Proceedings of the American Philosophical Society, Vol 10, n° 3 (jun. 15, 1960), p. 327 : "...if a son be sought for Caesar, it is Decimus, not Marcus."