Antony's army was never defeated, and indeed it was still a formidable force when Canidius was leading them on their way back. They surrendered because Antonius had abandoned them, not because they didn't think they could beat Augustus.
I'd say that their refusal to follow Canidius's orders to retreat to Macedonia and continue to fight, forcing him to abandon the army and flee to Alexandria, shows that they were neither formidable nor under any illusion about whether they had lost. Unless, that is, you're prepared to argue the same in the case of Lepidus and his troops. As it happens, we have evidence that some Antonian legionaries received grants of land: however, both common sense and the fact that large numbers receive nothing or are settled in Africa confirms that they were in no position to drive a hard bargain by threat of insurrection in the way that you imply.
Not all legions merged were given the title Gemina. For example, the Legio V Alaudae, raised by Caesar and fighting with Antony up until Actium, were merged with other units into Augustus' army. Yet they retained the name "Legio V Alaudae" rather than "Gemina".
This is another example of an Antonian legion having loyal troops transferred into it, though. What neither this legion nor X Gemina prove is that
he did a lot of his compression into 27 legions by merging legions together because
they weren't close to full strength. In fact, from the very fact that there are two Augustan legions which earn the subtitle
Gemina from being merged, we can actually surmise that it was a rare occurrence for loyal legions- otherwise it wouldn't have been notable enough in those two cases for the subtitle to be earned.
I'm not sure anyone in Italy was too inclined to do anything but keep their heads down in 42 BC too. This is right after the proscriptions remember, and Octavian does have dictatorial powers and a large army still.
Again, at the risk of stating the blindingly obvious, the thing about a
triumvirate is that there are three of them, and the thing about a
princeps is that there's one of him. The dissidents in Italy might have been inclined to keep their heads down
had it not been for the fact that the brother of a man just as powerful as Octavian is there to lead them. If you're a displaced landowner, or a veteran unhappy with your allotment, who exactly could you turn to in 30BC who isn't inextricably linked to the Augustan regime? Every public man of status left alive after the civil war either sided with him already, or is living on borrowed time after having been pardoned.
By 42 BC, the Roman army had just undergone a civil war that lasted from 49-44, and then another civil war starting in 43. That took a heavy tole on the legions, which itself was enough to probably knock them down from full strength.
Actually, what's more likely is that the number of campaigns between 50BC and 30BC result in there being a large number of veterans, or troops with at least some training, who are ready to come back to the colours. For instance, Octavian could call on the veterans of the 28 legions which were discharged after Phillipi to fill his ranks to fight Antony. This gradual process of increasingly militarising the population of Italy would explain how Caesar starts off with twelve legions and Augustus ends up with sixty to seventy.
while on paper it looks like Caesar and Antony and Octavian were levying these new full strength legions left and right, what is more likely happening is you have a large amount of under-strength legions.
However, the only reason that they would create legions is if they had the expectation of being able to bring them up to full strength in a sufficiently short length of time to use them in combat. It's not like the British in the Napoleonic wars, where the sale of commissions makes raising a new regiment more profitable than adding a new battalion to an existing regiment, and where regional designations like the 105th (Borough of Leeds) or the 129th (Gentlemen of Coventry) make these units more attractive to recruits. It doesn't really matter how full the new legions were (though, as I've argued below, the evidence is in favour of them either being full or much closer to being so that you've implied). What matters is that Caesar, Antony and Octavian looked at their financial resources and the manpower available and concluded that these new legions could be created quickly enough to help turn the course of the wars in which they were engaged. And those decisions provide us with very strong evidence for the ability of Rome to mobilise in the event of a threatened Gallic invasion.
I'm not saying the Romans didn't have the manpower for these numbers, just that logistically and economically speaking, it doesn't seem feasible that they actually raised all these troops in one location at one time.
But you can't just say "I don't think this seems feasible" and leave it at that: you have to look for evidence that supports or contradicts your supposition. For instance, you say that
if Augustus had to deal with the demobilization of 182,000 men he would have had far more problems than OTL.
Yet we know from the
Res Gestae that
when consul the fifth time [29BC]
I gave from my war-spoils to colonies of my soldiers each HS 1000 per man; about 120,000 men in the colonies received this triumphal public gift. It's clear that 180,000 is already in the correct ballpark: when you factor in that Augustus is talking specifically about his soldiers (
colonis militum meorum) suggesting that Antony's may have been excluded from the gift, and that some soldiers will have been given cash rather than land or sold their plots, the figure starts to look quite reasonable.
Should have mentioned it. It's a very nice site.