Most reliable Flash memory for an embedded Linux system?
If you want to see what is done inside, you have already answered your question : you can't use managed flash, your only solution is raw NAND flash.

As you want reliability, you can't have 2 pages corrupted when a write fails, this excludes MLC and TLC flash. Go for 2GB SLC NAND flash (BeNAND doesn't fit your requirements). It's expensive, but it's your requirement.

But let's be a little more pragmatic, you need your system to boot and to avoid to be fully corrupted by a sudden power loss. This excludes SD cards, but it allows eMMC : eMMC were specified for embedded environments. Manufacturers like Micron and Toshiba offer eMMC that you can partially use as pseudo SLC to increase reliability for your boot partition, that use buffering to reduce data loss and prevent filesystem corruption, without handling all the ECC correction/page relocation/Bad block handling/wear-leveling stuff by yourself, and, with JEDEC eMMC standard, you can learn about the number of available blocks, the mean/max age of your blocks, and you have dedicated commands to stop background operations, which allow you to perform a clean shutdown in about 20ms, instead of an indeterminate time with SD card. And this does not require a NAND controller with heavy ECC capabilities.