Translation:It took five days.
No, it is incorrect. "Itsuka" means "the fifth day of the month," while "go-nichi" means "five days"; the latter is the correct reading for this sentence, since we are talking about elapsed time rather than a date. This problem extends to a bunch of other exercises, unfortunately, and without the context afforded by the online course, I worry about the confusion it might cause.
Yes. In Japanese you count things differently. I recommend to read up on the different counting systems, they are quite complex. I really recommend an app called Human Japanese! It's more like a textbook but in app form <3
For me it said "itsukakakarimashita", I have no idea what to make of it.
You are right!
Either way is okay in case of days like this.
However, Like next is limited if it represents weeks.
It took five weeks.
５週間かかりました。←good ５週かかりました。←does not use much