I just pulled my sheave box this winter because most of the sheaves had stopped turning. I replaced all the sheaves. My sheave box has pretty much the same setup and was rigged as follows:
The lower jib halyard just ran around one large sheave (slightly larger than the rest). The second jib halyard ran over the next large pulley up, then to the lowest idler (~1") sheave. The spin halyard is run over the uppermost large sheave, and the uppermost idler sheave. The middle idler sheave is not used. It looks like it was put there with the intention of having a second spin halyard as well. There is space in the sheave box for a lower spin halyard large sheave.
I'm not sure about your small roller, but my sheave box did have a support between the two plates above the idler sheaves that the spin halyard runs below.
If you have any other questions, I could draw it for you and fax or e-mail it.
