Some fundamentals need to be understood here.

Firstly, A De Dion rear end is not an IRS since the hubs are joined by the de dion tube. The fact that you have a transaxle and half shafts does not make it an IRS.

Only advantage , albeit quite significant, is that you reduce the unsprung weight by have the differential attached to the body and not the wheels.

Secondly, Camber is fixed, there is no adjustment. It is built in. If you look carefully at the flanges to which the hubs bolt to they are pressed at an angle and thus have built in negative camber of a few degrees from the factory. Yes you can cut them off and re-attach them at some other angle if you wish.

I'm not sure why you label this transaxle as weak and rubbish. The 360 GLT runs a 2 litre motor. You are installing this in a Dato 1000. Of course depending on what you are doing with your engine and how you propose to use is will impact on the longevity of this axle or any other for that matter.

Perhaps go over to the Volvo 360 site and seek comments specific to your application. http://www.volvo300mania.com/forum-uk/