Bi-weekly(monthly) means twice a week(month) across the pond in the UK/Her Majesty's English, while bi-weekly(monthly) in the heathen states means once every two weeks(months/bimestrial).
I suppose it depends on where you are from and what you are trying to say. Probably just easier to say every other week :P
As an aside, you can technically get N7 cards in spectre packs. They are ultra rares but they do ever so rarely/occasionally pop up. I upgraded two of my N7 weapons this way. The N7 packs just make sure you get at least one N7.
Your mileage may vary and of course it could have been a bug but there you go. Don't lose hope :P
Yes the challenge is to continue playing afterr you realise the bi weekly ( is it bi monthly, never sure about correct usage here) challenge means it will take you two years to max out the N7 reward weapons.