I know you live in Japan, so do I. Work in a publishing company doing tech writing for some of the big tech companies in Japan (which I can't disclose, but I'm sure you can guess) so I see a lot of weird engineering Japanese every day.
As for the 部活 bit, it sounds odd any way you put it to Elizabeth, in my opinion. I don't think a native Japanese would answer I was at the club to the question did you join a club. On top of which the question is past continuous (部活に入っていたの), which is also weird for this situation. You cannot according to all logic be continously joining a club... So it would read better as 部活に入ったの?
Anyway, 部活 only refers to school clubS when speaking generally. When you're specific about it and use a club name such as バスケット部 or 映画部, you should only append 部 not 部活. 開いてください is more than odd, it's grammatically incorrect. As Elizabeth said, it's an intransitive verb and cannot take the object (something I knew from practice but didn't know formally, so there goes the illusion of my omniscience, if there was any confusion.)
I'm glad to know that you have the pages checked by a native Japanese speaker, but if two non-native speakers can pick at it like that, maybe you should make sure that he or she is not only giving it a quick glance instead of a thorough check... And don't get me wrong, I'm not saying this to insult you or your checker, it's just that I've run across this kind of behaviour before from Japanese...
***Update***
Thanks for the update. I found it rather funny...