Entertaining write-up. As a player who typically sets the AI at Immortal or Diety, I'd note the following as the single biggest turning-points, others may disagree:
1) Defensive wars should be fought with ranged units, not melee units. I wouldn't even have built swordsman until I was getting ready to go on the offense. As you learned when Portugal sent Ironclads after Frigates, ranged units are tactically superior to melee units because they can inflict harm without suffering harm. Melee units serve three purposes: defending your ranged units (including cities), seizing cities, and peripheral mop-up operations.
2) Don't make huge concessions for diplomacy's sake. You gave up a free luxury resource and entered into a pointless war to score diplomatic points, but the AI is not loyal enough to make this trade-off worth it.
3) When you go to war, mean it. Your wars against Venice, the Celts, the Aztecs, and Venice's city-state allies, as you eventually found out, were very costly for you and did not really yield any benefit. More so than ever before (thanks to how easy it is for your trade routes to get pillaged), declaring war at all has serious consequences. Declaring war against several city-states is incredibly dangerous, as at higher difficulty levels if you're enough of a warmonger, city-states will spontaneously declare war on you and never make peace. The more you make war, the more other civs will see you as a warmonger and feel no compunction about attacking you if you seem weak. If you don't have the resources to win a serious strategic objective in an offensive war, don't start it.
4) This is tied in to point 3, but you should have killed Venice when he proposed banning crabs. Not only was that the surest way of defeating his proposal (since a dead Venice has no city-state allies to vote), but it was good long-term strategy. Part of winning is preventing other people from winning. You need to keep an eye on who might beat you to the finish-line and intervene when necessary. Diplomatic Victory was enabled, and Venice was accruing lots and lots of city-state allies. Instead of a two-pronged war against the Aztecs and Venice both, you should have put everything into destroying Venice. With your formidable navy and the AI's complete lack of understanding of naval combat, it should have been easy to take the city if your navy was supported by ground troops.