1: was the phrase referring to the speculative nature of what he was embarking on (rolling dice) or referring to things being past the point of no return (casting metal in a die) or something else entirely?
2: how likely is it that these were his actual words (how credible are the sources).
Thanks.
In this case, it is believed that Caesar was quoting from the works of the Greek playwright Menander, who's work Caesar was supposedly fond of. The meaning of the phrase is thus derived from the greek phrase ἀνερρίφθω κύβος, which is generally used in the context of games of chance and can also mean "let the game be ventured".
Whether Caesar used it speculatively or not, it's not known, but authors who wrote about the event such as Plutarch and Suetonius generally use the phrase in the sense of going beyond the point of no return. Lucan, however, avoids the phrase and simply describes Caesar as "putting his trust in fate". Plutarch, in his work The Life of Caesar, notes that this was a common proverb at the time used by any who begin bold or dangerous undertakings.
The sources that consider the quote are largely from around at least a century after his death. Many of them are distorted with some degree of mythologizing and poetics, particularly Suetonius and Lucan, who even have supernatural elements to their retelling of the events. It is likely that the main sources of Caesar's life were also generally written with some regard to one another, with only a couple that were not influenced by the writings of Plutarch or subsequent authors. However, it is believed that the quote and "scene" of the crossing is largely derived by Plutarch and a few other authors from the writings of Asinius Pollio, a subordinate of Caesar at the time. It's not exactly the highest degree of credibility, particularly as Asinius Pollio is believed to have softened his account of Caesar to make him appear more in tune with legalistic Roman values. However, given Plutarch notes that it was a proverb that had entered common vernacular, it does not stretch the boundaries of credibility too much to believe that it was said in a speech, or in some other way.
What is generally agreed upon from what I've found, however, is that it is believed to have been said in Greek, rather than Latin. The Latin derivative comes from the works of Suetonius, where it's presented in it's corrupted form - "alea iacta est" - instead of "alea iacta esto".
Hope that helps!
Sources: Plutarch. Life of Caesar. Rondholz, A. Crossing the Rubicon. A historiographical study.