According to the Conversations API documentation in the case where a pre-action webhook fails to get a response:
(no response or timeout)
then
Conversations will publish the change unmodified after a series of retries; your messages will be delayed accordingly.
However it looks like the actual result is that Twilio returns an error to the mobile SDK when the webhook post times out.
Honestly the current response is the one I was hoping for, but since the documentation makes it seem like this is a bug, I just wanted clarification of what the expected result SHOULD be, before making any assumptions that break my stuff later on.