I knew a cat who was castrated through an injection to his testicles, this was around 1990 in Japan, done by a civilian vet (not a military vet). It took a couple weeks for his testicles to shrivel and fall off, after that he looked and acted like any neutered cat. Seemed like it worked and didnt have any major side effects, and was much cheaper than the surgical option, but on the other hand I assume there's a reason the method didn't become more popular.
I think I'd be way more leery of an injection that permanently sterilized a female cat, though, since everything is internal.
The only abortion method I know of is to have the cat spayed. I just did this for the little female I have right now, the only time I have aborted a pregnancy, but she was pregnant for the third time in one summer and was worn out and so thin. It costs a little more, but make sure the vet knows the cat is pregnant so they can be prepared, the arteries and veins are much more swollen and fragile then and they have to take precautions. she is putting on weight now and is fine, back to normal! All the oral sterilization methods for females I have read up on appear to be temporary as you have to give certain doses at regular intervals. As I had several females it didn't seem very reliable to me. I have had so many cats neutered and spayed now I have lost count, and have never lost one to the surgery itself.
I believe I heard of some kind of sterilization "pill" that's injected under the skin (about the same as microchipping) and which serves to temporarily sterilize the animal. I think they may have used it on great cats in captivity, but I haven't heard of any applications with domestic cats.
Hmm, if you had a very adventurous vet, they could maybe get Suprelorin for you: Long-term contraception in a small implant
It would be off-label, but then a lot of things used for cats are off-label just because the manufacturer doesn't want to do the trials for cats because they think it's not cost-effective. And it would probably cost more than a spay/neuter in the long run.
There is a mismatch shot for cats that can be given after an unwanted mating, but I don't know how far along it works. A spay is best if she's not a valuable breeding animal.