Actually, relays will quite frequently have to send T11 reports, so is it worth requiring primary instances to specifically ask for such reports?

This could be combined with information about the sender's view of the outstanding obligation at a particular time, so that instances can automatically identify whether they've got matching views of outstanding balances, and if not, when the discrepancy arose. Automatic resolution of the discrepancy should probably not be attempted; identification of who was at fault is likely to require humans to investigate.

Actually, to the extent that fixed-size payment paths incur fixed-duration processing by instances, this might actually help Big Brother trace a transaction through the network, since it knows it can associate a received message of a particular size with a similarly-sized message sent a specific duration later.

I'm no longer sure it's possible for fixed-size payment paths to fully conceal from the initiator's neighbours their proximity to the initiator, at least if we want to maintain the status-quo fact that a partial agreement represents an unbroken chain of genuine commitments all the way back to the initiator (which is important, so that participants can prefer partial agreements with more attractive prices, without risking ignoring all the partial agreements that could actually end up as part of a transaction).