Welcome to the forum
@DuchessDukelow
Don't know who give you the idea that I am good in keeping C&S, I like to chop those plants, but that don't make me an expert in keeping them.
The brown part in the last photo looks like stem rot, you may need to cut the brownish/soft part by 1 to 2 inch until you see the fresh and healthy plant. Beware of the sap, don't have any direct contact with your skin.
Those black spot maybe some damages from the spines during transport/packaging, or some insects are munching them. The yellowing part I am not familiar, possibly not enough sunlight. Sorry that I can not help much as I not familiar in keeping plants indoor, but I will forward this thread to the C&S forum, hope some other members can help.
And you have a beautiful plant there, it is blooming also
