Caesar wasn't worried about immunity through losing his imperium. That's an often-repeated myth I don't know the origins of, but Badian showed in a 1974 chapter that it wasn't Caesar's imperium that kept him safe. That chapter doesn't deal directly with imperium, and I don't know if he made the argument earlier, but he made it there at least.

Here's the text from Suetonius about the charges brought against Caesar, sometime between 58 and 56:

When at the close of his consulship the praetors Gaius Memmius and Lucius Domitius moved an inquiry into his conduct during the previous year, Caesar laid the matter before the senate; and when they failed to take it up, and three days had been wasted in fruitless wrangling, went off to his province. Whereupon his quaestor was at once arraigned on several counts, as a preliminary to his own impeachment. Presently he himself too was prosecuted by Lucius Antistius, tribune of the commons, and it was only by appealing to the whole college that he contrived not to be brought to trial, on the ground that he was absent on public service.

The college of tribunes didn't say having imperium was the reason Caesar couldn't be prosecuted; they said it was because he was absent on the business of the Roman state. Having imperium doesn't necessitate being outside of Rome. Consuls and praetors had imperium, and had lots to do inside the city since Sulla reformed the magistracies in 81. If it was simply imperium that provided immunity, there would be no need to worry about Caesar's absence.

The quaestor mentioned in the above passage was Vatinius, who was an ally of Caesar's in 59 as tribune of the plebs, and whom Caesar took to Gaul as a legate. He returned to Rome when he was charged, he appealed to the tribune Clodius, on the grounds of auxilium, and Clodius prevented the trial. Badian points out that Vatinius could have availed himself of the same claim as Caesar - that he was absent on official business - but he wanted to be aedile in 57, and that would mean being present in Rome for elections. He couldn't both claim to be absent and be present at the same time, so he used a different procedure to escape prosecution.

The reason Caesar was touchy about imperium in 50 and after 49 didn't have anything to do with prosecution. It was because he got screwed out of a triumph for his earlier Spanish command by being forced to stand for election for consul. That's why he secured the law allowing him to stand in absentia - it preserved his right to celebrate a triumph. His use of the consulship and the dictatorship after 49, Gardner suggests in her chapter in Blackwell's Companion to Julius Caesar, was a set of ad hoc arrangements in the context of civil war, rather than moves following a planned agenda.

Badian's article is in Polis and Imperium, edited by Evans.

EDIT: Oh, what would have happened if he had stepped down? If the law allowing him to stand for consul in absentia was respected, he would have ideal celebrated his triumph and then stood for consul. If they pushed around the election to make him forego a triumph like they had previously, he would have stood for election in absentia, then celebrated his triumph. As it was, he offered to step down so long as Pompey did also - a proposition a majority of the senate actually voted for. Only 22 senators rejected that plan. The consul Marcellus refused to accept the vote though. If that proposition had passed and been effective, he would presumably have come back to Rome, celebrated his triumph or stood for election, whichever happened first. Goldsworthy says Cato intended to prosecute, presumably on the earlier charges about his conduct in 59, but he doesn't cite any reason for this statement, and I don't know where it comes from, and as we saw with Vatinius, there were other grounds for avoiding prosecution in that circumstance.