Re 1: The trouble is that (by far) the most common thing that consumers do after buying X is to buy another X. The second most common is to buy another thing i

Re 1: The trouble is that (by far) the most common thing that consumers do after buying X is to buy another X. The second most common is to buy another thing in the same taxonomy as X (something very much like X, but slightly different). This is the case over a very broad range of taxonomies. So when you build a purchase propensity model, you often have to explicitly tell it not to recommend X.