It would be best to start with matching/flashcards for this lessom with all these new words
Why is Duo accepting so many alternate spellings? In this case, for example, وسعة is accepted.
Is this also the case in Arabic or is it simply a programming issue?
Programming issue but did you notice if it says (you have a typo) with the correct answer? Sometimes Duolingo accepts your answer if it has minor typos (I do it all the time when I'm not in the mood to type accents or umlauts).
The tips for the word "building" said none of the words among which to choose
Is there a tashdeed on "ي"?
Nope. (bináyatun wási3atun) [the audio is reading without grammatical rules like a dialect]
Say me fast