Got the job done—here's a recap/review.
First, to the question of is replacing ONLY the rear spring worthwhile? The answer is an absolute YES! On my first ride with the new spring, I immediately noticed a difference...and it was all good.
Though it's somewhat cliche...the bike absolutely felt more planted, more stable, and smoother over bumps (less rear-end pogoing). It didn't feel at all harsh or too stiff. I still need to take it out for a longer ride in the twisties...and I can't wait to ride it 2-up, where I know it'll be a MAJOR improvement.
As often is the case, getting the preload adjuster back into place (around the ABS box) was a LOT easier than getting it out—mainly because I knew what I was dealing with and knew exactly where/how to push/pull/bend/maneuver.
For the record, I removed ALL screws holding the ABS box in place (one on either side and two on the slanting steel bracket in front)—I think this was crucial. because removing all the screws lets you push the whole ABS box toward the rider's left side of the bike to make room to wiggle the adjuster through. In my case, it helped to have a friend on the other side of the bike pulling the ABS box out of the way while I maneuvered the preload adjuster through. Getting the 2 screws back into the slanted bracket was a pain...but not too bad (just required aligning the bracket and the whole carefully, then patiently getting the screw threaded with fingertips).
As for compressing the shock springs...after running all over town to find that NOBODY could (or was willing to do it), I said "Eff it" and resorted to the ratcheting tiedown straps method. This worked like a charm! The trick is to use 4 of them. I only used 2 initially and couldn't get the spring compressed enough (the ratchets became damn hard to move)...but with 4 it was easy-peasy. (The hardest/most time-consuming part was just threading the straps through the spring and ratchets.)
The nice thing about this technique is that you can compress the new spring off the shock with the straps...then just slide it over the shock (then remove the straps).
NOTE: If you try this technique, one end of the strap needs to be attached to the bottom of the ratcheting mechanism. In other words, most tiedown straps—as they come from the store—need to be butchered, because they typically have a short segment of strap stitched to the bottom of the mechanism with a metal hook. So I just cut that short strap/hook piece off...cut shorter lengths of strap, and tied one end to the bottom of the mechanism using two half-hitches. That worked fine. (Yeah, I butchered my tiedown straps, but they can easily be re-rigged again.)
Here are some pics:
And again, for the record...I weigh 225lbs, and bought a 13.4kg/mm (750 in/lb) spring from Racetech. Though many say that's too stiff, I'm not finding that to be the case at all. (I haven't checked sag yet...but just eyeballing it, it looks darn close with no preload at all.)