This is a point I, and I think others have made before. When more than one is involved in a lie then it becomes a conspiracy, and conspiracies always end up unraveling. The more people involved the quicker it unravels. Letters would indeed have flooded from Gaul and any 'inconsistencies' would have been utilized by Caesar's political opponents. Indeed, even the Catonic circle could not stop the senate from voting an unprecedented twenty days of thanksgiving for his campaigns. But lets step back here and take a look at the composition of Caesar's officer corp. Gruen provides perhaps the most in depth study of this time period, (thank you MPC for inspiring me to read this).
"The proconsul of Gaul, to be sure did not lack nobiles among his lieutenants, men of consular family who sought service abroad. They included the two sons of Crassus. One of them, P. Crassus, was prefect of Caesar's cavalry in 58 and continued to exercise important responsibilities in the succeeding two years...The other son M. Crassus, was Caesar's quaestor... C.Claudius Pulcher saw advantage in a post as a Caesarian legate...Q.Tullius Cicero, the orator's brother...Ser. Sulpicius Galba...In addition, four young nobiles, near the beginning of their public careers, are found as legati or praefecti in Gaul: M.Antonius, D. Junius Brutus Albinus, M. Junius Silanus, and C. Volacius Tullus..."
Though quite an impressive list Gruen goes on to say "In general, the contingent of nobiles under Caesar's charge is not conspicuous or imposing. Ten men of consular families fought in Gaul during Caesar's tenure."
Further.
"In rather larger numbers came representatives of new senatorial houses, families that first make an appearance at the beginning of the first century or were introduced into the ranks only after Sulla's expansion of the curia. Among the former may be reckoned legates, quaestors, and other junior officers like C. Antistius Reginus, C. Fabius, L. Minucius Basilius, L. Munatius Plancus, P. Sulpicius Rufus, Q. Titurius Sabinus, and G. Vibius Pansa..."
..and so on
This is likely just the tip of the iceberg. Can you imagine the volume of correspondence from Gaul? Or the scale of the 'conspiracy' if the numbers were systematically falsified? Didn't the Helvetian migration include a census written in greek?
On a side note/off topic in the same chapter Gruen states...
"The geographical origins of Caesar's legates and junior officers cannot in every case be documented. Occasionally, the literary and epigraphical sources give notice of an individuals home town. More often one is left to conjecture from the nomen or cognomen. But if the evidence is not always decisive for each individual, the cumulative effect is compelling: Caesar drew on most of Italy for his officer corps."
A prelude to Octavian's Tota Italia policy?