Why is it περισσότερο and not περισσότερα? Shouldn't it be the second because "more" is an adverb here?
The adverb is περισσότερο. Περισσότερα is the plural of the neutrum version of the adjective.
I wrote στην instead of στη and was marked wrong. Why?
It should have been accepted. It seems that this sentence is bugged and nowhere to be found in the incubator.