Ran is correct that both ממנו and מאיתנו are official because both appear in ancient writings. But using ממנו for the first person plural these days sounds wrong. Report it if you want, but use מאיתנו.
It'd be (technically) correct, since ממנו means both from him and from us (מאיתנו). However, it is more commonly used to indicate "from him". I do believe you can report your answer as "should be accepted".
I wrote את לומד מאיתנו and was told that I had 'spelling mistakes' (with the correction being the feminine form as shown above). I wish they would be consistent about which gender choices are accepted!