Unhappy [Q] Flashing no longer possible: Flash successful but system does not change


I can no longer flash anything to my GT-P5110, even factory reset has no effect. The internal storage seems to be read-only.

I think this is a longer story: About one year ago the stock firmware became incredibly slow, to the point that processes started to force close, then my user data was lost, and after factory reset the tablet went into endless boot loop.

So I decided to give CM10 a try and it worked flawlessly. The tablet was fast again. I even could upgrade to CM11 later. But around 2 month ago, only sometimes processes started to force-close or the tablet randomly rebooted itself - mostly during charging. I left it that way in the charger for some days because I had no time to bother with that issue. Then, I tried to factory reset my device only to find out that this has absolutely no effect. Installing app updates: No effect. Changing system settings: No effect. And apps start to force-close at an insane rate short after boot. The device is unusable. My could only shut down and reboot into recovery. I left the tablet lying around, turned off, out of charger, for 2 months. Now I tried to investigate:

When I "adb shell" into recovery and call dmesg, I see lots of these lines:

mmcblk0: timed out sending SET_BLOCK_COUNT command, card status 0x400d00
I think something is really broken with the internal storage. I tried to flash a newer recovery but it has no effect, even when done from download mode.

Factory reset sometimes fails with an error. Reinstalling CM fails with an error. Wiping cache seemingly succeeds but I'm sure it has no effect. I tried installing SlimRom instead which seemingly succeeds, too - but after reboot I'm greeted with the CM boot logo again.

I tried to flash stock firmware with Odin. It is very very slow, runs for about 15 minutes, then fails at around 50%. Of course: That "half flashed firmware" does in no way affect the device: It still boots CM11 "without problems", then a few seconds after finishing boot (showing home screen), force-closes start to pop-up.

What should I do? I suppose the device is out of warranty for a few weeks or months, now. And even when not: It has been flashed with CM, the triangle counter is not zero. I suppose that combination of problems (read-only/failing storage, non-stock firmware) voids warranty in a way that Samsung will deny any repair. If at least I could return the device to stock firmware things would be different maybe.

Any ideas what I could try? Is it possible that the internal storage is simply fried? fsck'ing the partitions shows no errors, just the data partition is unclean with unwritten journal data but all data is readable. So the internal storage has probably not died. It just sticks to read-only mode for whatever reason.

I've found some posts with similar problems but those seem to be about older Galaxy Tab 1 devices built 2011 or before.