Oh god, the author really dropped the ball on this one ...
A - Krajina (or whatever is the romanization of the word ) is just a Slav word for "Border" . Russia and other Slav countries have a lot of regions called that due to historical expansion ... heck, even Croatia has one
Ukraine is just "The border" if you go literal. A real Russian if someone said they came from Krajina would simply retort "Which one?"
B - The part of her being able to choose which bacteria she brings in and out ... nah. Even assuming that the Isekai god gave her full medical knowledge ( yeah, very likely [/sarcasm] ), there is the issue of gut fauna and flora, that are not harmful for the most part, but can become harmful in some situations, like in imunosupression contexts ( also, while my gut flora and fauna will not be harmful to me for the most part, they might be harmful to other people even if not in a Isekai world , so I assume that our resident loli is sterilizing her feces in the backround
). So if she decides to bring her gut fauna and flora, they might still become harmful ... and if she decides to wipe it, she will have some time of subnutrition ( because our gut flora and fauna makes some of the vitamins we need ) until she gets a new one and gets used to it . And then we start again , because her new gut flora and fauna might be as harmful as the old ...
TL
R Unless the Isekai God gave Mitsuha full knowledge of the future, she doesn't have enough intel to decide what bacteria are harmful or not, making her powers to select which bacteria to bring somewhat useless. And this is already assuming that she knows all of the possible medical knowledge there is , discovered or not ...
Yeah, there was some serious handwaving on this chapter :/