Comments relating to a few of the foregoing posts:
Caribbean 'black hole': Yes, I live in it. A friend in the UK, not 100 miles from GKA, with whom I have a Sunday sked on ham frequencies, complains bitterly that I will ruin his hearing as my signal is perpetually in the mud. Sunspots, and ERP seem to make little difference; it is, like the magnetic anomaly in the South Atlantic, just the way it is!
The Eastern Pacific is a really bad area for QSOs into Europe, however there doesn't seem to be much difficulty in the opposite direction - Asia. My conclusion is that this is due to the Rocky Mountains.
Although long-path to VIS from the Atlantic, did seem to work.
My memories of the Area Scheme (Appendix 9, published separately from the PMG Handbook, as I recall) are limited to on one occasion Mauritius telling me to contact GKL (as it was then) directly as my QTC was too long!
Having escaped from the confines the UK flag and getting decent levels of transmitter power, and receivers that stayed on frequency, I quite enjoyed the direct contacts with the world's HF stations and all of their idiosyncracies.
(One example: Choshi/JCS would accept 1030N 11845E as three words, Nagasaki/JOS would make it four: I could go on, but it would be way off-topic).
The worst HF stations, to me, were Chalna (or was it actually at Khulna, I forget), and Chittagong. The problem was that if, failing to raise either of them, one used Karachi (this was before East Pakistan became Bangladesh), the QTC used to travel by camel to it's destination so that the ship would actually arrive before the ETA advice!