I don't think we should have to be that specific with translating to the language we're learning with (English here). I'd wish I didn't have to use the correct form of is/are in English either. I'm not an English speaker and I'm learning Hungarian. It's very annoying to make an English mistake, that doesn't affect the meaning, which means I get a wrong on that one.
Well, part of the reason I wrote a post from the contributor's perspective a couple of months ago...
I used to think the same way and when acting like a learner, I still kinda do. It's annoying not English natives are basically discriminated in English courses that they only take because there isn't any other course in a language they know to any usable extent and it's boring all courses turn into one big English lesson at some point.
However, you should be aware of a couple of things. You seem to know it already that accepted sentences are basically created by the people who maintain the content (volunteers, at the end of the day but that doesn't matter now). Now, adding an indefinite, never settling set of incorrect English sentences is tedious and awkward, eventually not worth it. And there is another point that I personally believe in - one can't actually check whether the learner understood the sentence correctly, based on an incorrect solution. It's hard to make sure a mistake was "English only". After all, we have all reason to assume learners who take this course know English better than Hungarian. If their English solution is incorrect, it's a bit far-fetched to imply they got the Hungarian meaning right.
Having said that, with certain things, we need to be less concerned about English. A recent example - changing most "can't see/hear/find something" sentences to "don't/doesn't ..." sentences. It might be less common, in fact it may very well be much less common - but it should be understandable and it's still better than having a lot of people aiming for a literal "can't" sentence in Hungarian which would be completely off. In English, "can't ..." variants are still accepted, just not used when translating to Hungarian. That's how a tradeoff looks like.
No, it doesn't really work, it sounds very odd. Saying "szépen köszönöm" is not entirely incorrect grammatically, but it would mean something like "It is in a nice way that I thank you". That is, you want to emphasise "nicely", i.e. you're thanking nicely and not in any other way.
See you later should be accepted as well, since “viszlát” is short for “viszontlátásra”
It appears http://www.grammarphobia.com/blog/2009/06/thank-you-kindly.html that it would.
Depends on who you are speaking to. "Bye" can mean a simply greeting in hungarian. Example: "We'll speak tomorrow, bye!" can translate to "Holnap beszélünk, szia!"
"Viszlát" is a more formal variant which you will most likely use when interacting with elderly people or your teachers, etc... Yeah it's kinda strange.