Why "suomalaista" and not "suomalainen"? Because of the plural? Or some other rule here?
There is no plural. Numerals (except "yksi") are treated as amounts in Finnish, and amounts trigger partitive case in singular form.
Definitely will be helpful to get some lesson tips to explain all this.
Well this is still in beta. I'm fairly certain that they'll have all the relevant stuff sorted out by the time the course leaves beta stage.
Is there a timescale set for that, do you know?