if you have the 124 with the 603 like me, here is a vacuum diagram (1st) that may be useful and two that cover turbo function (2nd) and the turbo set-up (3rd) . you seem to have the basic operation of the tubo correct.
the alda, the way i think of it, is a fuel mixure device, it richens the fuel ratio corresponding to the pressure inside the intake manifold. as pressure builds the mixutre richens. there is a pressurised line going from the intake into a solenoid (in diagram, 'Y30') then into the alda device. if this is the relay you mention, then it serves to cut off boost pressure in the event of an overboost condition by closing and having the alda produce a mixture relative to low or no boost pressure being sensed. BTW this solenoid gets coked with soot and is a common cause for lost boost or poor performance.
as for the EGR, it is commonly held here that it can be removed or its vacuum line plugged with out negative affect. there is hovwever an ARV (air recirculation valve) that is on the turbo, but seeing that you still have the old trap it is probably advisable to keep it hooked up since it is responsible for making the charged air hotter so that the trap can 'regenerate' properly.
there are abundant posts dealing with ARV and EGR "modification", check these out.
