True... I must admit I never use mouse-look...

The mouse-look spoofing is to make believe to the server and (more specifically) to the scripts running on it, that you are in mouse-look while you are not (this is to make some stupid, stubborn, invasive scripts work while keeping your freedom as to what view you want to have on your screen), but it won't cause the viewer to stop rendering objects behind your avatar's mouse-look FOV (since you are actually not in mouse-look), which is what I suggested this trick for...
Because Firestorm didn't (yet ?) implement my fixes to LL's buggy code, so they suffer the same bugs as LL's viewers (failure to update moving, rotating, rescaling child primitives) instead of having the vehicle bug (for now, you got one bug or the other: there's no modern viewer that manages to render things properly in all cases: only antiquated viewers (v1.23 and older) do it right !).
This said, I've got a new idea for a possible fix: no promise, but I'll have a try at it and see what happens...