I think there are some things that aren't really clear:
1) There isn't anything about a specific timeframe to place icons, how will doing BN activity going to be accounted? For example, as long as I don't participate in 7 mapcycles, there won't be any score calculated, aside from that I'd be getting from mods. So I could be getting steady score by doing mods and avoid participating in ranking (even safe icons). Maybe you don't consider it a flaw at all, you can keep helping maps by modding and only iconing when you really, really see something worth it, instead of being forced to meet monthly goals, but this needs to be cleared.
2) The calculation uses your last 7 rankscores/contributions, would the calculation be triggered from the 7th map onward (7th map, 8th map, 9th map) or every 7th map (7th map, 14th map, 21th map, though this option would be a bit lenient, as in some modes like CTB participating in that much maps takes longer than in other modes). In the first case, let's assume someone messes up and gets 3 DQs in a row, like this (O is something that got ranked, X something that got DQed):
1st Calculation: OOOXXXO
2nd Calculation: OOXXXOO
3rd Calculation: OXXXOOO
4th Calculation: XXXOOOO
Since "this score is added to your overall "Score Balance" - much like before in the monthly ranking", this "fail" would be added to your score 4 times, which sounds kind of a rough penalty. Maybe you have thought of this, but adding some weightings on the score based on their position on the list (the latest weighting more in the calculation) will balance this, i.e., on the 1st calculation they will have a big weight (since they are recent fails), once this example BN starts doing better, the fails get a smaller weighting and their punishment on the latest calculation isn't that heavy (getting -100, -75, -50, -25 score on each calculation instead of -100, -100, -100 and -100 [disclaimer: random numbers]).