I think at least on a spoken level both are acceptable. In Ukrainian we always have many variations of saying a word, especially if there is a possibility of making it gentler, smaller, cuter etc. For example, донька --> до́нечка, also доця, доня, дочка, дочечка (not sure about this one, never used it, might be just Russian)
Whoa, no, I think that's a good question to the experts here :) My guess is that it could be simply divided into the usual east-west influence: до́нька sounds more similar to до́чка in Russian (the same stress position), so regions who got more USSR influence historically tend to pronounce it like that. And the western parts of the country, which kept it more pure and/or had Polish influence (or eastern Ukrainian-lovers, haha) tend to favour донька́. But it's just a guess.
Why does the Ukrainian language need that extra letter for the sound "i" ?? My native language is a slavic one, and we have only a one letter for the "i" sound, the letter "И"...so far i cant really distinguish those two letters "I" and "И", they sound exactly the same :(
Yes, those are tricky but they are two different sounds indeed, that's we we have two separate letters for them. I see you are learning Russian too and it's the same there as well: и vs ы.
Don't worry, however, with some practice you will learn to hear the difference after a while. And I'm also pretty sure that most of the time, if not always, people will actually understand what you mean, even when you use a wrong sound, so it's not a big deal really.
The ь letter doesn't have a sound per se but it rather signals that the preceding consonant has to be palatalized ("softened", we call it).
I see you are learning Portuguese, so you can read the нь combination just like nh in Portuguese "minha" or "aranha". Or if you are familiar with Spanish, the нь combination sounds like Spanish ñ, so you would spell the word "донька" like "doñca" in Spanish.