i have to warn that you'll face many skepticism since those who download the backup need to trust you.
No trust is needed. The torrent equipped bitcoin node still queries its peers for the most work header.
But if the file is corrupted or contain invalid block, then you wasted some time and few hundred GB of internet data.
But whenever this node needs a block with a certain hash, it can just see if such a block is present in the torrent and use that instead of asking its peers for it.
I don't get this part of your statement. OP seems to backup Bitcoin Core files where one .blk file which contains data from multiple blocks.