Salve Publi Fabi

What you said is correct, assuming Caesar had backed down at that point. The only reason the Senate thought it could defy Caesar or prosecute him was because they had Pompeius and his army behind them. Civil War was also avoidable if the Senate had acceded to Caesar's request to run in abstentia. However that is not the only point in time that Civil War could have been avoided. After crossing the Rubicon, and even later after Pompeius had departed from Italy, Caesar sent envoys to Pompeius to resolve their differences. Had Pompeius accepted there would not have been a Civil War and some other arrangement would had to have been made. I am assuming that had that been the case then the arrangement would have seen both Caesar and Pompeius as consular colleagues. The combined strength of the Senate could not oppose the combination of Caesar and Pompeius, and thus there would not have been a Civil War. So even after fighting had begun, had the two of them come to a settlement, the Senate was helpless.