Just a couple of observations. The idea that multi-wire branch cct's are not being used very much is not true. It is still an economical way of dealing with two circuits fed from the panel that are located far enough away to warrant the savings or convienance of not installing two separate home runs. The split 15A receptacle is still popular because of the total wattage available versus 2 20A. The only reason the 20A became popular was the necessity to use GFCI protected receptacles near the sinks. As was noted in previous discussions on the board, you cannot protect a split receptacle using a common neutal with a GFCI breaker or plug.
My advise . . . do the job right. Only then will you sleep well because if there is ever a fire as a result of the bedroom circuit being impinged by a cord against a headboard etc you place yourself in a very sticky situation legally and morally. Abandon one of the wires on the 3 conductor in the attic JB, put the receptacles on one circuit and install a QO AFCI and be done with it. Obviously in order to do that you will need to not have put too many devices in the bedrooms. Most bedrooms you will get away with 5 receptacles and a light (although I don't like putting the light on the AFCI cct see below). The bedrooms are probably the lightest loading of anywhere so I wouldn't worry about your idea of keeping the circuit lightly loaded.
I don't buy the idea of a staple causing problems with AFCI unless it had damaged the conductors as it is not a complete loop or bonded in any way and therefore should not have any effect on the sine wave. What often wreaks havoc with AFCI's is arc producing loads including switches that are not making good contact, vaccuum cleaners, or wore out lamp bases etc. My understanding is that the AFCI is able to sense the distortion of the wave caused by these things and then trips.
As usual you will be getting opinions from Canadians as well as Americans. The codes are similar but not the same. My perspective is Canadian Electrical Code. Good luck, it is a pain when you find something like this after everything is finished.
