May I please ask you:
I started saying the bracha of “al hamichya”. I was half way through when I realized that I should have said “boreh nefashos”. Is it sufficient to stop where I was and to just say “boreh nefashos”, or do I have to start over with “Boruch atah Hashem …” ?
Yes, you can stop where you are and say borei nefashos and continue from there. You do not have to start the bracha again. In fact, even if you finished saying “al hamichya” and you corrected yourself right away (toch kdei dibur) you don’t say another bracha. The mistake that you made isn’t considered a hefsek in the bracha. See sources.
O:CH 209-2, M:B 209-5, Sharei Habracha 11-11 ftnt. 32. It is important to note that the halacha would be different if one is bentching min hatorah see O:CH 209-3, Sharei Habracha 11-13 ftnt. 36.