The most heinous of the treason laws broken by Caesar during his proconsulship were those enacted by Sulla, specifically the ones that prevented him from personally declaring war on another without prior approval of the Senate (done to prevent Triumph seeking generals from starting wars on their own, exactly what Caesar was). Caesar was abundantly guilty of this, in any trial he would be found guilty and exiled, which is what his political enemies planned to do. To prevent it, he needed to be elected consul again, in order to push specific legislation through during that year that would legalize everything he had previously done, to make him untouchable. His enemies knew he planned on doing that, which is why they didn't let him run for consul in absentia, he had to surrender his army. Had he done so, as a private citizen, and without Crassus (dead) and Pompeius (sided with the Optimates), Caesar might not have enough clout to guarantee winning the election or avoiding a quick trial.
Now backtracking a bit, if Sulla didn't march on Rome, it would mean Marius would have gotten his way at having Sulla removed from command of the Mithridatic War. Supposing Marius actually survived the war, health wise, and was victorious, it still would have meant an entrenched Senate split between Optimates and Populares. Meaning another spark most likely have started a Civil War, Rome was do for one.
But Pompeius and Crassus were Sulla's lieutenants, which is why they prospered. Both of them were crucial to Caesar's rise to power. He never would have been given the Pro-consulship to fight in Gaul, nor had it extended, if not for the Triumvirate.