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were all animals vegetarian/vegan before the fall?
Yes; they were created this way (Genesis 1:30), and will be this way again in the New Creation (Isaiah 65:17-25).
Did carnivores evolve from vegetarian ancestors?
Be careful with the word "evolve". Evolutionism is an anti-theistic fallacy that has never been demonstrated and the only "evidence" for it is a bunch of assertions founded in the dogma that God doesn't exist.
One possibility is that carnivores arose from herbivores via the process of natural selection (which has been observed and is accepted by YECs), which is the process of species changing their attributes within the available genetic diversity of their Kind. This diversity was already present at Creation; God, in his wisdom and foreknowledge, would have allowed for carnivory from the start and provided certain animals with the genetic potential. Indeed, all animals (AFAIK) can eat meat (yes, even ruminants!), and will do so if it is available, but some are better adapted to obtaining it than others.
Another possibility is that some animals were changed by God as part of the Curse. One possible evidence in support of this is the apparent change to "the serpent"; Genesis 3:14 suggests that God may have immediately altered the anatomy of "the serpent", in which case, perhaps other changes were made at the same time. However, even if this occurred, additional changes as discussed in the previous paragraph may have also occurred.
p.s. Naturalists believe — contrary to the laws of thermodynamics and information theory, because they have no other choice that does not invalidate their religion — that such information can spontaneously come into existence via "random mutations". There are significant problems with this conjecture, but the only alternative is Intelligent Design, which is anathema to Naturalists.
p.p.s. Yes, I used the R-word. Naturalism is a religion (or philosophy, if you prefer, but the difference between religion and philosophy is pedantic at best) whose unassailable dogma is "there is no God". Evolutionism is a necessary result of Naturalism, although some Christians have been duped into also accepting it through various social pressures.
Were carnivores created by God after the fall?
No. On the seventh day, God rested from his work of Creation (Genesis 2:2). God's work of Creation was completed at the end of the sixth day.
Were carnivores the result of genetic manipulation performed by fallen angels after the fall?
I am not aware of any evidence for this hypothesis, not of any significant groups that believe this. OTOH, I don't offhand feel entirely confident in categorically ruling it out. My personal belief is "no", that carnivores arose via natural selection, but I can't "prove" it.
I should also add that there are various records of cats — supposedly obligate carnivores — thriving on a vegan diet, in accordance with Isaiah 65:25. This would seem to support that even cats were, at some point, herbivores.
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