I have a real dumb idea for Egypt in the TL that I might/might not write. This is the same scenario where Keraunos' assassination of Seleucus fails and he moves on to incorporate Macedon to his empire. It's kinda long.
Ptolemy's proxies in Epirus and the cities of Greece are quickly dealt with by the Seleucids. The Seleucid king Antiochus notice Ptolemy II's involvement and begin courting his stepbrother Magas in Cyrene. He offers his daughter's hand in marriage, military support and recognition of his rule over Cyrene should he declare independence. Magas interprets Antiochus' words as an opportunity to seize Egypt for himself.
He declares war on Ptolemy in 280 BC a few years earlier than OTL (274 BC), now confident that he can win with Antiochus' support. There is no Marmaridae rebellion to distract him from committing fully to his war. The Seleucid king is a concern but Magas is the more immediate threat as he's closer to Alexandria so Ptolemy commands an army, some of whom contain several thousand Celt mercenaries, many of them survivors of the failed TTL Celtic invasion of Seleucid Macedon. Marching miles of open desert, the Gaulish mercenaries conspire to unseat Ptolemy and create a Celtic kingdom along the Nile. He doesn't catch wind of their conspiracy and the Celtic mercenaries murder him and his guards at night. The army awakes to hear the news of their dead king.
Shit turns cray. Magas and Antiochus both take the news with delight. The former moves east to occupy the Libyan cities and the latter dispatches his army to move southward to seize Phoenicia and Judea. Egypt becomes a free for all as different factions are formed, each one commanded his wife Arsinoe, siblings, concubines and other distant relations, each vying to put their own candidate on the Egyptian throne. The Celts bide their time waiting for their opportunity to arise. The native Egyptian population chafes under Greek rule and are making noise of a revolt. The priesthood, seen as collaborators complicit with Greek rule, fails to calm tensions. Large bands of peasants and machimoi deserters begin raiding Greek-owned estates, murdering the kleruchs and their families. The periphery is soon lost. As Seleucid and Cyrenaican armies draw near, the factions come to a decision that they must cut their losses and make peace.
The First Syrian War comes to an end.
Ptolemy's lands in southern Syria and Anatolia are ceded to the Seleucid Empire; the new frontier being at the western edges of the Sinai peninsula. Cyprus recognizes Antiochus as overlord but retains some autonomy: the island is divided into several minor kingdoms and republics. As the native Cypriot houses had been wiped out (to my knowledge), they are given to the sons of his most loyal followers to rule in his stead. Due to Seleucid pressure, the Egyptian court factions agree to crown Magas as Pharaoh. As an adopted member of the Ptolemaic family, he becomes Ptolemy III Magas. His wife Apama becomes Queen of Egypt.
As the new sovereign of Egypt, Ptolemy III accepts Antiochus' conditions and pays a crushing war indemnity. The Egyptian navy is cut to a bare minimum of 50 warships and their Meroitic-derived African war elephants would be added to the large Seleucid arsenal of the Indian breed. Egypt would no longer be allowed to have its own war elephants. Its army would be limited and Egypt can no longer go to war independently without the consent of the Seleucid king. Or else. Backed into a corner and unable to say no, Ptolemy III agreed to the terms. The war comes to an official end and Antiochus returns to his capital but retains an army at Gaza.
Magas had achieved his goals but with a heavy price. Egypt is now crippled with an indemnity that would take years to pay and is now a vassal state of the Seleucid Empire. Egypt had lost all its possessions outside Africa. He rules as Pharaoh but not by his own actions. He's seen as a puppet of Antiochus by the army and hated by Ptolemy's family as an outsider unworthy of sitting on the throne.
To pay for the indemnity, he increases taxes on the kleruchs who pass it to their Egyptian peasants. The natives soon begin to flee the estates in droves to join the bands of brigands afflicting the countryside. Unable to send troops to defend every estate, he delegates handling of defense to the sepat/nomes whose rulers begin to plot an independent course. In Alexandria, the factions plot against Ptolemy and each other once again.
Ptolemy III Magas' rule would be opposed to several competing factions in the Egyptian court, each supporting a different contender to the pharaonic throne and doing whatever it takes to get it.
One would be led by Arsinoe II: she was the second wife of Ptolemy II and his sister in OTL but Ptolemy's reign is cut here ITTL so he doesn't divorce his first wife and marry again. Her candidate to the throne would be her son by Lysimachus, Ptolemy Epigonos. Arsinoe seems to have had an significant influence in the court if her brother was compelled to marry her during OTL's First Syrian War. Ptolemy Epigonos is also believed to have been Ptolemy II's original intended heir prior to the former's rebellion. I have no doubt that she could rally support behind him. It might not matter but he's Lysimachus' last surviving son.
Epirus is going to fall either way to Seleucid rule so Pyrrhus' sons and any exiles would find refuge in Egypt. Leading them would be Ptolemy Pýrrou, son of Pyrrhus. He's more interested in supporting a candidate that would revitalize Ptolemaic interventionism in the Aegean and restore him to his father's throne but he has benefits in seizing the Egyptian throne. Through his mother Antigone, he is also a grandson of Ptolemy I and has more of a claim than Magas.
Ptolemy Euergetes is son of Ptolemy II by his first (and TTL only) wife Arsinoe I of Egypt. Like the other Ptolemies, he's a grandson of Ptolemy I but he is also the eldest son of Ptolemy II and considered a legitimate heir to the throne. Enough said.
You have the Celtic mercenary faction that is waiting behind in the shadows, waiting to strike. Since there's no names for the conspirators behind the OTL Celtic revolt against Ptolemy II, I would have to use creative license and use a survivor or two from the Celtic invasion of Macedon to lead the faction. They'd support a weak candidate that they can boss around and use until they can get rid of him - maybe Ptolemy Andromachou, son of Ptolemy II by his mistress Bilistiche.
Increasingly paranoid (no help provided by the court factions, feeding him information of assassinations plots by the other, both real and fabricated), Ptolemy III Magas seizes control of the large estates of his political enemies amongst the kleruchs. Most of them are granted to his most loyal supporters, the Cyrenaicans. To placate unrest in the Egyptian population, he divides some of the estates and grants small land allotments to the natives. It doesn't work and increases resentment of the Egyptians, solidifying the belief that the Ptolemies, no matter who's king, were not fit to rule. It
pits the disfranchised kleruchs towards the monarchy that they supported and pushes them to the arms of the factions.
Landless, some migrate to the Seleucid Empire where they are granted new land grants in Syria and Asia Minor. Others move westward to the cities of Carthage and Syracuse where they become soldiers for hire, generals - important factors in the wars between Rome and Carthage. Most, without land or coin to their name, become sell-swords for the growing armies of the nomarchs and southern kleruchoi, facing attacks by bands of brigands.
No longer safe touring the country, he holes himself in Alexandria, surrounded by his ministers who attempt to make the best out of a deteriorating predicament. Crippled with the war indemnity, he sends his pregnant wife Apama to Seleucia to plead to her father to alleviate the debt and provide troops to quell the uprisings. Out of concern for his daughter's safety, he obliges and reduces the debt owed by Egypt to the Seleucid Empire. He sends a token force of a few thousand men to Alexandria, as he can't stretch his forces then.
He has an uprising in Persia to destroy and no expense can be spared in destroying them.
The Egyptian court and Alexandrian garrison voices complaint of the presence of the Seleucid force to the King. Seeing treachery, he dismisses them and on the urging of the Queen, the Seleucid troops serve as their replacements. The troops are undisciplined and unruly, extorting the people of Alexandria for their "services." The wealthy Greeks and Jews (to a lesser extent) are able to pay the Seleucid guards to leave them alone. The poor Egyptians have little to nothing so you can imagine what they'll do. The different factions begin using the Alexandrian's frustrations to their advantage.
Ptolemy growing more mad, his Cyrenaican ministers subtly take the reigns of government from him and establish the Synderion, picking its members from the military, landowning elite and a select few prominent citizens (to shut them up and make them feel like they have a say). The different court factions use the Synderion as a place of demagoguery, filling its seats with their supporters, denouncing the rule of Egypt's Mad King. Despite this, the Synderion is a suitable valve to let off steam against the King and the ministers can focus on rebuilding Egypt. They vote to remove the Seleucid soldiers from their city; the Seleucid soldiers refused the Synderion's orders and the commander, claiming to act on behalf of the King, orders them to be arrested. Many are captured and executed publicly ; among them are former Queen Arsinoe I of Egypt and her son Ptolemy Euergetes.
The factions begin to coalesce. Ptolemy Epigonos' candidacy as an alternative King become stronger with Euergetes' execution. Pýrrou drops his claim to the throne in exchange for Epigonos' promise to support him to conquer Epirus. Epigonos marries Pýrrou's sister Olympias as a guarantee. Epigonos cuts a deal with Bolgios, a prominent kleruch (and one of the few not displaced by the Mad King) and de facto leader of the Egyptian Celts, to gather his army and come to Memphis. Bolgios' loyalty was bought with promises of more land for him and his men and their families.