'Kuna embe jikoni' should be the right translation.
Both work. Mna is less common than kuna but it emphasises that the mango is really inside the kitchen. (You could also achieve this by using ndani ya jiko.)
Could you give a similar example in English (sure it's hard but it would be helpful) thanks!
Not exactly. Jiko also means stove/cooker. Ndani ya jiko may mean inside the stove. Mna/Kuna/Pana should all work equally.
I also find it weird that they reject "kuna" and yet "kuna" is more used than "mna"