Why not apply DefaultMouseSpeed of 0 to SendInput at all times, also when SendInput reverts to SendEvent? This way the scripts behavior is more consistent and less dependent on the environment it's running in.SetDefaultMouseSpeed is ignored for SendInput/Play modes; they move the mouse instantaneously (except when SendInput reverts to SendEvent; also, SetMouseDelay has a mode that applies to SendPlay).
For other settings reverting to SendEvent does already change certain other parameters. For example a modified KeyDelay (-1) is used when SendInput reverts to SendEvent, Applying a mousespeed of 0 when SendInput reverts to SendEvent keeps the behavior more consistent with situations where it does not revert.
In the mean time it would help to also mention this behavior in the documentation under SendInput, as now it is only mentioned under SetDefaultMouseSpeed. But when making modifications to Send mode, people that read the documentation will look under SendInput and are not triggered to consider SetDefaultMouseSpeed behavior changes.
Context: I have been searching for why a script was working perfectly on my computer but not on someone else's. It took me a while to discover that on the other computer SetDefaultMouseSpeed was ignored. When working on the script, SetDefaultMouseSpeed was always applied with SendInput, so I never thought of it that SetDefaultMouseSpeed would normally not apply to SendInput. It was only when I specifically started to look for it in the documentation that I learned about this behavior.