Because it's vastly different with upgrade routes and movements. Some major notes are:
No, you could make the save file read-only which is located in Documents\CAPCOM\DEVILMAYCRY4SPECIALEDITION\ This will pervent the game overwriting the file each time it attempts to auto-save.
Majority of the strats for this game rely on just playing well enough. Which means, at one point you will probably theorycraft something that won't lose time, compared to the strats and cycles seen in the runs.
At certain point yes, DMC4 will become a dynamic run that requires runner to adapt several situation. Bianco Angelo in Mission 6 is a good example.
You could use Quick Drive or Distortion but, it's important to have this core skill in the run as mastering it will shave over 2 minutes.