Not sure if I'm missing something, but I'm curious if (how) this will work if the sponsored transaction's feerate is so low that it has been largely evicted from mempools due to fee pressure, and is too low to be widely accepted when re-broadcast? It seems to me that the following requirement

>1. The Sponsor Vector's entry must be present in the mempool

means that you enter a catch-22 where the sponsor transaction cannot be broadcast because the sponsored transaction is not in the mempool, and the sponsored transaction cannot be (re-)broadcast because the fee is too low. This requirement might therefore need to be revised.

There is of course no global mempool, but RBF by its nature would still work in this case, by replacing the transaction if it exists and inserting it if it does not.

--AC