I didn't yet use NAND flash for storage, I only evaluated the technology in component identification, but opted for NOR flash in the respective project. I mainly refer to general information, that's availiable from NAND flash vendors. They give suggestion for ECC, partly providing software libraries (C-code or HDL).
1. NAND flash is programmed page wise, single bytes aren't accesible for write. As far as I understand, consecutive programming of pages is required after erasing a block. Some devices allow multiple prgramming operations of a page (only '1' to '0', of course).
2. I already answered the question, typically 100k erase cycles. But some blocks may fail before, NAND storage isn't safe without considering this fact.