Well - according to their website, they may charge me a fee if my warranty turns out to be void.
Luckily I was able to revert the repartitioning by editing the partition table via fdisk using rupor's sd recovery. I also reflashed the boot loader and recovery kernel to disable adb / rooting.
But all partitions above mmcblk2p7 where inaccessible. My only chance was to copy back image dumps via dd directly to the mmcblk2 device with the seek option set to the partition starting sector. Unfortunately, dd stops when a write error occurs. I wish it would skip bad blocks and continue writing to the good ones.
Anyways - I didn't want to spend the whole afternoon messing with it and finally sent it to a Sony Repair Center.
If they do a dump of the internal flash, e.g. via ddrescue, they'll definitely find bytes indicating that the reader was rooted, as well as maybe my Google and Amazon Kindle account data. But as you said - I doubt they'll do that much effort.
Fingers crossed
Meanwhile I got myself a
Kobo Mini for a third of the price I could have bought a PRS-T2 and I even consider selling the PRST1 if it ever comes back repaired, because I like the smaller form factor of the Kobo and I also appreciate the stable Linux firmware - it's all about reading ebooks and the book shelve feature is awesome, especially in conjunction with Calibre.
It boots up much faster and there is no more messing around with crashing Android Apps, wake_locks causing battery drain, fixing app permissions, repartitioning, etc. And the internal flash is a µSD-Card that can be easily dumped for backup and replaced with a bigger one
The only things I miss are PDF reflow, pinch-to-zoom and hardware buttons.
But displaying complicated PDF with technical drawings is way faster than on the PRS-T1!!