Thanks for writing this up!
One thing on the APOAS section. You noted that APOAS signatures could be replayed against other UTXOs at the same vault address, and recommend unique keys per vault.
Replaying the signature in a separate transaction only unvaults the second deposit, which is harmless, but I think the half-spend problem is worth mentioning as well: that both deposits can be spent in the same transaction. Since APOAS commits to neither the input count nor the input index, a transaction with two vault inputs and a single fixed output verifies at both inputs. 200k sats in, 99k out, 101k to the miner.
BIP-119 gives this as the reason CTV commits to both: committing to the index “makes it safer to design wallet vault contracts without half-spend vulnerabilities”.
Perhaps this is helpful: https://covenants.diy/g/nhcg5hYdxO. I’ve wired up an APOAS vault and a CTV vault side by side with the same two deposits. The APOAS transaction is complete and verifies at both inputs (discarding half the payment); CTV refuses the second input because the index is in the hash.
The missing commitments do buy fee flexibility, but they create the half-spend exposure, so never reusing a vault address is a requirement rather than a recommendation