(the only way to replace a transaction is Replace-By-Fee but this
implies the transaction that IS TO BE REPLACED has a certain flag set,
and it is optional).

1. full rbf is becoming standard. tx without full rbf can just be rejected as a part of the sabu protocol

2. that's fine. watchtowers can be used to fix this that only have the ability to stop attacks in progress. guarantee transactions can include a small watchtower fee for your favorite always-on watchtower service

3. all parties need to maintain sabu state (it's just another mempool)