can't put subreports in subreports. Crystal only goes down 1 level. Main->subreport. You can have multiple subreports in a main, but a subreport cannot contain a subreport.
shared variables are easy. in a formula, just declare and start using them. usually you will need 3 formulae to do everything. 1 formula to display, 1 to set to zero at the top of the group and 1 to accumulate. The last does all the work, and so is a bit trickier than the rest.
HTH