For point 2, you can't just expose a smaller block to the next layer - everything is designed to work with 512-byte logical blocks. You could possibly do it if you were willing to sacrifice half your disk space by pairing every 512-byte data block with a 512-byte authenticator block, but...
...the other issue is that you need a way to atomically update the data block and authenticator together. This is a similar problem to the "RAID write hole", and doing this at the block layer requires even more overhead - something equivalent to RAID write intent bitmaps.
The filesystem, on the other hand, already tends to have a journal, log-structure or COW rules that allows some form of atomic transactional updates. The encryption authenticator can piggy-back on this.
SSDs have fairly large page sizes and erase block sizes, so you're often reading and writing a lot more than necessary, anyway. A clever implementation might be able to take advantage of that to scatter the MAC throughout the drive without too much impact on performance or longevity.
...the other issue is that you need a way to atomically update the data block and authenticator together. This is a similar problem to the "RAID write hole", and doing this at the block layer requires even more overhead - something equivalent to RAID write intent bitmaps.
The filesystem, on the other hand, already tends to have a journal, log-structure or COW rules that allows some form of atomic transactional updates. The encryption authenticator can piggy-back on this.