Not so hard a retrofit as I completed it within a couple hours as I finally got around to installing the fold-down seats after sitting in storage for six months. The only difficulty was getting the longer left seat fully engaged onto the long pivot rod of the shorter right seat: the side brackets should be installed first on the sides of the seats (otherwise getting a Torx bit to tighten their screws would be almost impossible) but they make the seats very tight in their installed positions and inhibit wiggle room to easily slide the left seat onto the pivot rod.
To answer my own question about the bracket: the center support bracket needs to have the rounded side down to match the welded body bracket which has a rounded bottom edge to it.
I opted to install the two-piece center seat belt in order to prevent the original one-piece seat belt from interfering with large, bulky cargo with the seats down. Here are some notes for those also performing this part of the retrofit:
The rear window shelf needs to be removed and modified to install the plastic end-buckle stowage holder (aka Cap, Lock Tongue, PN 51468208863). There should be many other DIY's that detail removing the shelf, but be careful prying out the speaker cover; the aging plastic tabs will probably break off unless you know exactly where each tab is to apply pressure via a trim removal tool (I broke the side tabs).
The shelf is pre-scored for the plastic stowage holder. I simply used a box cutter (with new blade) and poked through from the bottom following the scoring.
Then carefully cleaned up the opening from the top and installed the holder. Tightening the its plastic nut will gradually pull the holder nice and flush onto the surface of the shelf.
I'm now enjoying the newly expanded cargo capacity that I had sorely missed with my E46 with its fold-down seats. It seems this option wasn't as widely selected/available with the E90's, being mostly sold in more northernly/wintery locales.