You can definately stack radius doubling effects.  (obviously leading to some quite large radii)  As for the number of secondary casters you can get, Sam can attract 10 1st level adept acolytes and 1 2nd level cleric to help her with the (secondary) ritual to protect the temple while the planar convergence thing goes off.<br/><br/>Writing up a ritual schema takes an average of one hour per spell level, but if you already know the spell, it takes half as long.  If you've cast the ritual before, it also takes one sixth less long, to a minimum time of five minutes per spell level.