1) "Front upper spring mount": IPD sells these, I would think they mean strut mount, right? IPD says this is a weak part of the suspension system, has been upgraded by Volvo, yadda yadda. Should I just go ahead and replace these? BTW, my car only has 55K on it (but I am still fed up with the harsh ride).

Mine was shot at 40k so do get them replaced or you'll have to get the entire strut assembly removed again when they fail and it'll be soon judging by Volvo's quality.

2) "Upper Mounting Plate": Dan at volvoparts.com says these are replaced on an as-needed basis only. But is this the same part as the upper strut mount, or is it something entirely different?

These metal plates are mounted above the rubber spring mounts (item 1) and should last through two spring mounts before failing.

3) Bushings: I know that Sin likes the Powerflex bushings, I am wondering if I should replace my current bushings at this time as well. How many bushings would I need?

I just got a set of powerflex lower a-arm bushings replacing the stock bushings (48k)and so far no harsher ride felt. Moreover these powerflex stuff will last definitely longer than stock replacement.