OK I got it. At the root (cough) of this it seems that Apple (or the folks they outsource development to) don't follow any icon naming and path rules.
cjm3113 (thanks cjm) had actually posted the answer to the independent GOCdock above- but on first attempt I dismissed the link as dead. Was I ever wrong about that.
Hack that Phone is a very helpful, well-organized, and donation-worthy reference that I was unaware of until now. I'm in no way affiliated, but I've got it prominently bookmarked now, and I highly recommend it.
Armed with valuable information from there, I proceeded thusly:
1. I reset my previous meddling by removing and reinstalling Glass orb Color theme in Cydia (again). It would have been better while figuring this out to backup and rename files and directories on the iPhone as I proceeded, but I try to keep the bloat down on my phone, and don't keep anything on it that I
think I don't need.
2. As on previous attempts, from /var/stash/Themes.xxxxxx/Glass Orb Color.theme/Icons/ I dropped Safari.png iPod.png Phone.png Mail.png and Text.png onto my desktop.
3. As on previous attempts, I carefully selected all files remaining in the above folder and deleted them. Then I returned the 5 survivors to their folder (including Text.png for posterity).
4. Back on the desktop, I renamed that copy of Text.png to icon.png
5. Navigating back to /Applications/MobileSMS.app/ I gave the default icon.png a backup name, dubbing it icon.png.GORB
6. Now I dropped the the file formerly known as Text.png that I renamed in Step 4 into the MobileSMS.app folder.
7. Resprang, and now I am enjoying a GOCdock with default icons everywhere but the dock.
This experience has taught me why theme and tutorial developers deserve our support now and then. There are many details and time invested in these tweaks, but many people have a fleeting span of attention about the nuts-and-bolts of app and theme development. I believe that instead of wringing our hands over the Apple business model (or other corporate revenue structures) those of us who desire friendlier alternatives should cultivate the habit of contributing monetarily to the evolution of open markets.
I don't make my own living developing, tweaking, or marketing software. But I do believe in being kind to folks you meet, and in complimenting and rewarding the people I encounter who serve others for a living, whenever it's possible and appropriate. I believe that such habits can and will change the world for the better, if understood and undertaken collectively.
Satisfied with this tweak, I probably won't be re-arranging the looks of my iPhone now for a long time. So, now is the right time for me to send a small amount of cash to
ToyVan and
MCJ who enabled me to enjoy the look of my iPhone homespace again. As I keep finding such good info here at macrumors, I'll remember to donate here as well. Thanks!
Sorry for the long posts, everyone.