Flash memory in TO is (up to my knowledge) SK hynix eMMC4.5 which manufacturer specifies, for density of 4GB, 2.4TB total bytes written before EOL.
eMMC on its own does not track write cycles like standard drives and does not have mechanism like smart. You can use mmc extcsd read /dev/mmcblk0 from package mmc-utils to take a peak in to amount of used reserved blocks. That is field EXT_CSD_DEVICE_LIFE_TIME_EST. Value is in range of percents so 0x01 is from 0%-10% blocks used, 0x02 is from 10%-20% and so on. When amount of reserved block used is close to 100% then NAND is pretty much EOL.
You should know that this is not linear. This is secondary statistics and tells you nothing about complete wear. Common wear is an avalanche effect. Nothing is happening for a long time and then all blocks starts failing at once. Meaning percentage of used reserved blocks might start rising very quickly.
Note that you need TOS 4.0+ to see required field. Version of mmc-utils in TOS 3.x is not capable to read it.
We have also in pipeline tool called healtcheck that is intended as a monitoring tool for potential problems on router.
Non that we know about. There might be and user can easily create one just by miss-configuring some standard application. Nonetheless all applications we know about that by design write to FS are pointed to /srv and storage plugin can be used to mount /srv to external storage.
That is pretty much impossible. It is 153 ball FBGA. I am not saying that is can’t be done and you can found companies and individuals that are able to do that but it is almost not worth it. Unless you know someone who is able to do this kind of repair for cheap then it is not worth it. And because you have to heat up board to desolder and solder chip you are also risking some other malfunction.