The text you quote says nothing about what values are used for IV. To me it looks like each block has a static name, probably based on its offset in the addres

The text you quote says nothing about what values are used for IV. To me it looks like each block has a static name, probably based on its offset in the address space of the virtual block device, and that there's nothing telling the block pointing at a block which version of the block to use. So you can replace block N with any known good version of that block. To point at specific versions of a block you'd have to point at something like hash(encrypted_block) or hash(block_offset || block_version) instead, but then you'd have to update pointers all the way up to the root for every edit made anywhere in the FS.