Securely erasing flash memory is surprisingly hard.
>Whole-disk wiping techniques faired only slightly better with SSD media. In the most extreme case, one unnamed SSD model still stored 1 percent of its 1 GB of data even after 20 sequential overwrite passes on the entire device.
Actually what if one of the blocks used for the critical data was deemed degraded and removed from the pool and replaced with a reserve before it was overwritten. No way to delete then although expensive to retrieve it might be possible.
Depends on the capability of your opponent and the consequences if your guess is wrong.
Flash uses pretty large block sizes (look up specs for write size and erase size for common flash media; it's often 4KB writes+128KB erases), and the wear leveling algorithm can unpredictably shuffle the writes around. Your final paragraph is probably the only safe way to know an SD card's internal flash is unrecoverable, followed by physical destruction.
for i in $(seq 1 1000); do if=/dev/urandom of=/dev/sdc bs=$((1024 * 1024)) count=(SD size in MB); done