一週間は いっしゅうかん です
Does 間 act as a counter here? what would the meaning of 一週かかります。be?
Yes, basically. 間（かん） is used to specify an interval or period of time. It can be used after a number of days, months, minutes, etc.
Why the not ケ?
ヶ, ケ, か, カ, and ヵ are all shortened forms of 箇, which is only used for months and other counters, but never for weeks.
間 here means the "duration of".
I have never heard anyone pronounce 一週間 as いちしゅうかん - isn't いっしゅうかん more natural?...
yes, ive already heard いっしゅかん too when learning the days of the week... im confused
Why does it add the third symbol? Can't you just only use the kanji for "one" and "week"
It seems that Duolingo only showed me the first comment, my apologies for the double post
When I was in Japan they used "shūka" for number of weeks (so "ka" not "kan"), I don't know if this is just casual usage in conversation or not though.
Why is 間 needed here? Just week isn't enough?
It's not, 週間 is the word for week, when it's used for a period of time. It's not used for everything, but it is for week, second, sometimes months, years, and more.