OK, the "theory" is that one possible reason why evaps are failing on 850s is because copper material from the blower motor brush area is getting on the evap. "They" then think that this causes some electrical/chemical type reaction when combined with dirt and moisture from incoming air when this "stew" is on the evap and THAT's what causing evap failures. Now.............I have never seen physical evidence of what looks like small holes burned through the exterior or any 850 evap (and I've replaced/inspected MANY), I usually see the dye leaking through the seams at the bottom of the evaps and to me, it "looks like" it's leaking from the inside out. I find the copper brush idea hard to believe (but creative ;)) and think that the real problem is more to do with moisture in the system reacting with PAG oil (which becomes acidic) and burning through from the inside. I've "heard" that that idea has been found to be true on other car makes too. It's then very important to replace the accumulator and PROPERLY evacuate the system using an AC vacuum pump whenever servicing/replacing any part of system, especially if it's been open for a while (like hole in condensor, line and no freon).
Adding a blower motor air restrictor won't cause the blower motor to overheat as they feel there's too much airflow to it as is.
Since evaporators have been failing (a fairly recent occurance in Volvos BUT ever since introducing 134a), there's been MANY "theories" on why this has been happening but most of these ideas I simply don't believe. At first they even said "acid rain's causing it" BUT older Volvo R12 cars drive in the same acid rain (and longer too) w/o evap failures.
I personally wouldn't bother with the idea of adding the tape for the blower motor housing UNLESS I was already there for a reason, like had a reason to remove the blower motor.