Yes. The formal/respectful second-person pronouns are identical to the third-person plural, except that you capitalize them. They follow the same rules for verb conjugation as well. That's why Sie sind willkommen. is ambiguous: it means both "They are welcome" and "You, sir/madam, are welcome."
In my experience, Duolingo will only mark something as a typo if it does not constitute a totally different word. So, if you misspell "world" as "worlf" or "qorld", or "wrld," that's fine, but if you type "word," you will be marked wrong, and if you type "wled," or something where more than just a letter or two is off, you will also be marked wrong. Extra or missing spaces are normally marked as typos as well. All told, if "danke shon" is exactly what you wrote, there are three typos: two minor ones and and one serious one. I would expect Duolingo to mark it like this:
dankeshön = missing letter = typo
danke schön = extra space = typo
dankeschon = missing umlaut = typo
danke schon = "thank you already" = wrong
danke shön = extra space & missing letter = typo? wrong? (I'm not sure here.)
danke shon = "thank you already" where "already" has a typo = wrong.
In any case, I'd be surprised if "dankeschön" were not accepted, as that is what I have been taught for "thank you (formal)" from other sources.
Well, "schön" (or "schoen" where umlauts are not possible) and "schon" are two very different words, so that may be why.
Or it could be that Duolingo wants to distinguish between a simple "thank you" and a "thank you very much" . . . I would think of "danke schön" as being the latter, and this question asks about the former.
This post is rather off-topic . . . anyway, the easiness of learning a language will never be a good motivating factor. Don't pick a language because it is easy, pick a language because you want to learn it, then stick with it until you have. (Also, which language is easier to learn is very subjective, and varies depending on your native language.)
I am French and am learning German. Both have very tough grammars, but I think German is tougher because of the cases. French is known worldwide as a beautiful and fancy language, while German is often seen as an ugly and harsh language. I personally adore how German sounds! :) just go with your favorite. You might want to consider Spanish if you're into an easy language (not saying its easy!! Its just easier than German and French).