Does the emission of copious blood from the anus during defecation—which only appears upon defecation—affect the validity of the fast? And is it permissible to fast or is a make-up required, especially with a scheduled surgery in mid-Ramadan?

What you have mentioned does not affect your fast. You must fast Ramadan, unless it is highly probable that you cannot endure fasting based on personal experience, or by the decision of a trustworthy Muslim doctor that fasting would harm you. In such a case, you are permitted to break your fast, and you must make up the days after recovery.