Damn those are arbitrary rules, most of them. They sound like the author first wrote the story for the manga and then came up with rules that would conform to the story. They are always convenient and rarely inconvenient.
I surely hope there would appear a rule that would be inconvenient. Right now the single source of tension in the series is the guessing game between Light and N and M. This is essentially why I hope the new shinigami shown at the end of the ep will come and shuffle the cards some. As great and filled with nice surprises as this has been, it still flows a bit too smoothly and doesn't pick too many totally unexpected and out of ordinary paths, considering this is a series populated with mythical shinigamis and all.