You seem to assume that all support for the First Citizen in Rome with the biggest Auctoritas is unconditional no matter the threat posed by external factors, such as the threat of civil war. Was everyone 100% confident that Pompey would defeat Caesar? Consul may not be de facto the most important position, but if Consuls choose to break with their backer, they had real constitutional authority to not exercise a veto for example.
When there is no danger, the First Citizen is the most powerful with authority outstripping Consuls. Put the Republic in a crisis, make the First Citizen obstinate risking civil war, and there's a risk some of his supporters will defy him just this once, since it is a small concession to Caesar that hardly puts him in a position to dominate Roman politics. (Pompey is also hardly the shrewdest politician, nor someone to never forgive slights)
OTL in my understanding the decisive factor for being unable to avoid civil war ultimately was the Consuls being unswayed by Cicero (due to Cato) and vowing to veto any attempt by the Senate to make this small concession to Caesar to avoid civil war. Otherwise there was enough support from moderates and Optimates with common sense to secure a compromise.
I of course agree that, OTL, Gaius Claudius consul for 50, and both consuls for 49, played a crucial part in bringing the political conflict to meltdown point because they would not compromise and give-up their and their allies’ obsession about causing Caesar’s downfall.
However it was precisely in crisis situations that first citizens most visibly asserted their Auctoritas. Much more than in quite times.
And this was quite logical because being first citizen did not derive from prestige but from real power and hard resources.
What made Pompey the most powerful man in Rome for a generation was not his prestige but his unmatched resources. He was quite clumsy in the Senate and on the forum. But he had and could summon far more resources than anybody else because he was by far the main patron in most of the Roman provinces, especially in Spain and in the East.
Prestige was but a consequence of power and of real resources.
That’s why in crisis times, nobody cared much about speeches or about official magistrates. The really most powerful men, be they magistrates in office or not, we’re the decisive actors. A Cicero could still make great speeches and go and privately talk with other nobles and with the most powerful ones (which he was not), these men did what they wanted whatever Cicero said. Pompey anyway said Cicero he thought war was unavoidable because he did not want to avoid war at the price of giving-up his claim to supremacy over Caesar.
Caesar’s father in law Piso, although censor in office, could try what he would to cool things down.
The civil war was not the result of Pompey’s will alone. If Pompey alone had wanted to bring a crisis in his relationship with Caesar to military meltdown point, he could not have succeeded in doing so.
The civil war became possible and happened because Pompey allied himself again with the core of the optimates who wanted Caesar’s downfall but who could not reach their goal on their own because Caesar was too big for them to swallow alone.
The consuls alone, in 50 and 49, had no such weight to decide on their own. It was only because they had Pompey’s backing that they overrode the tribunitian vetos and the 370-22 vote of the Senate in favor of both Pompey and Caesar laying down their extraordinary commands.
It was the same during Caesar’s first consulate. Caesar did what he did, overriding vetos and delaying tactics, because he knew he had the actual support of the then 2 most powerful men in Rome : Pompey and Crassus who supported Caesar for the precise reason that Caesar had the political craft and guts to do what it took to pass the measures that Pompey and Crassus were longing for but that the optimates blocked.
Which explains Caesar’s statement in 50/49 that he had made enemies in advancing Pompey’s interests, in fighting those who then were Pompey’s enemies, and then Pompey dropped him and allied with his former enemies against him, Caesar.
But this was not a matter of legality and office positions as magistrates. Pompey and the optimates had allied to form a new faction that, although a small minority in the Senate, was the most powerful on the Roman political stage and all the provinces except Gauls. And they would have their goals fulfilled whatever it took, in a different way but in the same logic as Marius maneuvered in 88 in order to destroy Sulla by snatching the eastern command away from Sulla’s hands.
Same situation and same result : civil war because neither Sulla nor Caesar would let himself be destroyed without fighting with their standing armies.
Although surprised by the quickness of Caesar’s military moves in January 49, Pompey was not surprised by the outburst of the war. He had all calculated it. He knew he had more resources than Caesar and that time would almost certainly buy him a victory by attrition. After Caesar’s defeat at Dyrrachium, Caesar was almost finished. It is only Pompey’s fatal strategic mistakes that gave Caesar the opportunity to turn the tide of the war by offering him at Pharsalus the pitched battle that would enable him to play his best and last asset : his tactical superiority and his better (although only half the size of Pompey’s) army.
