For most people, their location and flight patterns practically determine the single best FFP to join. Fortunately, it's becoming increasingly less so on a number of long-haul routes, and one can then have the luxury of considering such finer details.

An airline or FFP having a specially-crafted policy to disallow newsletter opt-out signals unwillingness to accommodate customers even where it costs nothing to do so. Would it be wrong to assume even more of the same is likely to show up whenever money is involved, i.e. in practically any other situation? It might be a factor then, particularly if only one FFP does this.

This is something I've encountered as well on a number of occasions. Interestingly, the "subscribe" button rarely seems to suffer from similar problems. From my experience though, in such cases customer service is quick to unsubscribe people manually when requested to do so.

Perhaps. I've never claimed anything to the contrary.