It should be common knowledge that you can’t just bring prescribed medication into a foreign country. There is even plenty of OTC medication you can’t bring into some countries.
Heck, in many places you can’t even bring fruits, meat, or dairy. Not doing your due diligence before traveling is on the person traveling.
I don’t think she deserves to be in prison for that and I would assume if relations with Russia at the time were better she would have just been denied entry/fined.
But breaking the law is still entirely on her and returning such a high profile prisoner in exchange for her mistake stings.
That depends a lot on the exact prison. Because many prisons don’t follow guidelines and provide less food than they are supposed to.
In theory, inmates are supposed to get between 2,400-2,700 calories a day. The actual amount might be closer to 2,000 or even below that. For a small person that might be enough to sustain themselves but bigger people will lose weight.
Unless a prisoner has money. Then they can use the commissary store to buy additional food and eat as much as they can afford.
And that’s for the entire western world. Some countries are better at enforcing guidelines than others but every country has a few prisons that are run terrible.