Good point re redirecting remote scripts, and yes, you can still execute JS in the page's context using a content script. However, we'd lose some control here. With <script> injection we can at least [make an attempt to] track those elements. We can track remote script redirection to some degree. With direct response body modification we have no idea what's going on.
And yes, malicious extensions can always be malicious with the sort of privileges they need to run webRequest. Modifying response bodies is more subtle though.
I think that read-mode is ok from this perspective though?