As I see it, if they allowed him to run for consul in absentia and granted him a triumph, as was his due, the greater majority of his legions would be paid out/disbanded after his triumph. A few would remain in Gaul to keep the peace.

I'm sure they believed/knew he would win, just as I'm sure they would be able to saddle him with another inimical colleague in the consulship.

What specifically did they fear he would accomplish in his consulship? Even if he extended his imperium (immunity from prosecution) by 1-2 years (he probably would get another provincial command) there was almost no chance of him rallying the political support with Pompeys opposition for a 5-year pro-consulship over 3 provinces.

After his pro-consulship expired, he immunity would be lost, his legions long disbanded, a private citizen like any other.

If they feared him marching his legions on Rome, why provoke him to take that exact course of action?