Is the blood and bloody discharge that appears after a miscarriage considered menstruation or istihada (post-partum bleeding), and are missed prayers obligatory to make up? If so, how are they to be made up, and what about the ghusl (ritual bath) for them?

This blood you see is considered menses as long as it occurs at a time when it is possible for it to be menses. You must stop praying and abstain from all that a menstruating woman abstains from until the bleeding stops, then you perform ghusl and pray.

As for making up missed prayers, you must make them up as much as you are able, provided it does not harm your body or livelihood. Your making up should be immediate whenever possible, and in order, as a precaution. Ghusl is not obligatory for making up prayers; rather, you perform wudu.