It seems like in the meantime I was able to fix my PRS-T1 (although I was very close to taking a ride to the local Sony shop and nailing it on their front door).
I ultimately ended up rebuilding all partitions according to these instructions here:
https://wiki.mobileread.com/wiki/PRST...itioning_Guide
One of the unnerving parts was that I found it impossible to get the correct sizes under Linux - but Putty under Windows finally did the trick.
After rebooting, I simply dd'ed the partitions again you gave me, there were no errors and I've had no problems ever since (knocking on wood).
Bottom line: "Something" screwed up my partitions. Interesting idea that SONY themselves might have changed the partition setup over time, but then again I bought my device over a year ago, so I doubt it (if that lead to all the problems I encountered, it would basically mean that it's unrootable with the standard packages out there, and I assume someone would have reported it by now).
As for the dictionary pack - yes, I flashed that one as well.
Will I root again: Gut says no, brain says now that I know it all, I would be stupid not to.
For now, however, I'm not touching anything for the next couple of months to make sure the problems are really gone.